Okay, I don't know why things are working now when they weren't before, but I've uninstalled and reinstalled the USB-MIDI driver several times to verify that it's recognizing my YPT-400 each time. I've also verified that it's actually working in Ableton Live.
To briefly recap the issue, I had the driver installed already and it was working perfectly with my PSR-E433 and PSR-E443. But when I bought a used YPT-400 (equivalent to the PSR-E403) the driver wouldn't recognize the YPT-400, and Windows said that the necessary driver for the YPT-400 was not installed. For a year or more I'd occasionally seen other people post about not being able to get Yamaha's USB-MIDI driver to work with their older keyboards, so when it happened to me too, I assumed it must be some sort of compatibility issue.
It appears that the "trick" to get it to work is simply to make sure the YPT-400 (or other model) is already connected to the computer, turned on, and showing up as an available device (except without a driver) in the Windows Control Panel. Then you should be able to install the driver and it should recognize the keyboard with no problem. Even though the steps are very simple, I've written up some rather lengthy detailed instructions. The basic steps are numbered (1), (2), (3), etc., and then they're broken down into more detailed steps numbered (1a), (1b), (1c), (2a), (2b), etc.-- so if you don't need all of the excruciating detail, just read the main steps.
I've also added some comments about using the keyboard in Ableton Live after you've got things set up, but they definitely aren't meant to take the place of reading the Live reference manual or taking a class to learn how to use Live. Anywho, without further ado...
________________________
Note: The following is written for Windows 10 and the Yamaha YPT-400 keyboard. Some of the details might differ slightly for other versions of Windows or other models of Yamaha keyboards.
(0) If you haven't installed any version of the Yamaha USB-MIDI driver yet, skip to step (2).
(1) If you've previously installed a version of the Yamaha USB-MIDI driver but it won't recognize your Yamaha YPT-400 keyboard (or other Yamaha model), you will need to uninstall the driver, as follows:
(1a) Open the "Control Panel" in Windows, select "Programs," select "Programs and Features," wait for the list of programs and updates to finish being populated, scroll down as needed, select "Yamaha USB-MIDI Driver," and click "Uninstall."
(1b) Confirm that you want to uninstall the application and all its features. The uninstall process should take less than a minute.
(1c) The driver will still be shown in the list, so you will need to refresh the "Programs and Features" list to verify that the driver is no longer shown.
(2) Download and unpack the newest version of the Yamaha USB-MIDI driver for your specific operating system, as follows:
(2a) Go to Yamaha's web site for your country (e.g., "http://usa.yamaha.com"), select "Support," select "Downloads," enter your keyboard model (e.g., "YPT-400"), and click "Search."
(2b) The "Search Results" will list only the "Related manuals" for the keyboard. To see the related drivers, click the link for your keyboard, which is found just above the "Related manuals" table (e.g., "-> YPT-400").
(2c) To make sure you download the correct driver, click the "Filter Software and Drivers by Operating System" dropdown box and be careful to select your specific operating system (e.g., "Windows 10 64bit"). You may also want to click on the "Type" dropdown box and select "Driver." If two or more drivers are listed for your operating system, select the newest one as determined by the version number, not by the "Last Update" date (e.g., "V3.1.4" is obviously newer than "V3.1.3," regardless of what their "Last Update" dates might happen to be).
(2d) Agree to the "License Agreement," click the button to download the requested file, and extract its contents to a convenient location. (You shouldn't need to download the "USB-MIDI Driver Installation Guide," as it should be included in the file you just downloaded.)
(3) Connect the keyboard and install the driver, as follows:
(3a) With the keyboard turned off, plug the squarish end of a standard USB 2.0 "printer cable" into the keyboard's "USB" port (if it has two "USB" ports, use the one that says "TO HOST"). Plug the flattened end of the cable into a standard USB 2.0 port on your computer (USB hubs and/or USB 3.0 ports will not work).
(3b) Turn on the keyboard. A window should appear on the computer that says "Installing YAMAHA Portatone" (or something similar), although it may be minimized to the Task Bar. After it closes itself, open the "Control Panel," select "Hardware and Sound," select "Devices and Printers," and verify that the "YAMAHA Portatone" (or whatever it's called in your case) is shown in the "Unspecified" category.
(3c) Go to where you extracted the USB-MIDI driver package, find the "setup.exe" file, and double-click it. Verify that you want to install the program, agree to the "License Agreement," and begin the installation. It should take less than a minute. (If it says that no compatible device was found, you'll need to start over from square 1 and keep trying until it works.)
(3d) Go back to the "Control Panel" and select "Hardware and Sound." You should now see the "Yamaha USB-MIDI Driver (WDM)" listed. If you go to the "Devices and Printers" again, you should see that the "YAMAHA Portatone" device is no longer shown in the "Unspecified" category and is now shown in the "Devices" category. Close the "Control Panel."
(4) With the keyboard turned on, set it up in Ableton Live, as follows:
(4a) Start Ableton Live, click on the "Options" menu, and select "Preferences."
(4b) In the "Preferences" window, click on the "MIDI Sync" tab. On the first available (unused) row, click on the dropdown box in the "Input" column and select "Yamaha Portatone-1" (or whatever it's called in your case). Then click on the dropdown box in the "Output" column and select "Yamaha Portatone-1."
(4c) In the "MIDI Ports" section, click on the button that's at the intersection of the "Track" column and the "Input: Yamaha Portatone-1" row to turn it on. (If it already says "On," just leave it alone.) Do the same thing in the "Output: Yamaha Portatone-1" row.
(4d) You may also want to turn on the buttons in the "Sync" and "Remote" columns, but they're optional. The "Sync" option lets the keyboard and Live exchange MIDI clock pulses to keep tempo with each other-- "Input/Sync/On" tells Live to use the keyboard's MIDI clock (you should see a blinking indicator in the upper left corner of Live's screen), while "Output/Sync/On" tells Live to send its MIDI clock to the keyboard (you would also need to turn on the "ExtClock" option in the keyboard's "Function" menu); thus, you wouldn't normally use both "Sync" options at the same time for the same MIDI device. The "Remote" option lets the keyboard and Live exchange "remote control" information; you would normally use this option with a MIDI control surface, although the keyboard does have two "Live Control" knobs that can send control information to Live.
(4e) Close the "Preferences" window when you're done.
Once you get to this point, you can select the "Yamaha Portatone-1" as the "MIDI From" device on any MIDI track where you want to record the MIDI data coming from the keyboard. Or, if you want Live to send MIDI data to the keyboard (i.e., to play it like a "player piano"), you can select the "Yamaha Portatone-1" as the "MIDI To" device on one or more MIDI tracks.
MIDI data has no audio content, so you won't be able to hear any of the MIDI data that you record from the keyboard unless the MIDI track is assigned to one of Live's instruments, or its MIDI output is being sent to the "Microsoft GS Wavetable Synth" (or other MIDI "soft synth"). In both cases you won't be hearing the keyboard's sounds, so a third option is to select the "Yamaha Portatone-1" for both the "MIDI From" and "MIDI To" devices on a given MIDI track.
Note that we're talking about when you play back a MIDI track that you've recorded, since obviously you should be able to hear what you're playing on the keyboard while you're in the process of making the recording.
If you use the keyboard as both an input and output device at the same time, the MIDI data coming from the keyboard will be echoed back to the keyboard by Live, so you might want to turn off the "Local" option in the keyboard's "Function" menu. Doing that will "silence" the keyboard so that playing on it won't generate any sounds, but the keyboard will still play the sounds for any MIDI data it's receiving over its USB-MIDI connection. (Just don't forget to turn the "Local" option back on later!)
Since recording the keyboard's MIDI data won't capture any of the sounds coming from the keyboard, a typical workflow would be to record the keyboard's MIDI data to one or more MIDI tracks, then edit the MIDI data to correct any incorrect notes, tidy up the timing, add MIDI controller events in an "automation lane," etc. Once the MIDI tracks are to your liking, set up an audio track to capture the keyboard's audio output, play back the MIDI tracks and send them to the keyboard, and record the keyboard's audio while Live is playing it like a player piano. That's assuming you want to record the keyboard's sounds. You can also use the keyboard as a keyboard controller and record the sounds coming from Live's instruments, or you can do both-- use the keyboard's sounds for some tracks, and use Live's instruments for other tracks.
Another type of workflow would be to use the keyboard's built-in song recorder to record one or more tracks on the keyboard itself, then play back the recording on the keyboard and record its audio output in Live. However, the onboard song recorder in Yamaha's PSR-E and YPT keyboards doesn't allow the editing of individual notes, so this approach probably works best for people who can play a keyboard without making mistakes, and wouldn't work as well for people who aren't as skillful.
A variation of this would be to purchase or create a MIDI song file, load it into the keyboard, play it back on the keyboard, and record the audio output in Live. One possible drawback to using this approach is that MIDI song files are covered by copyright laws, although you could use this approach if you're creating your own MIDI files of original compositions.
I still need to verify that it's actually working, but this is the most progress I've made on it since I bought the YPT-400 used late last year, and I was so excited to get this far that I had to post about it right away!