I have this new computer with windows 8.1. I plugged in my Yamaha ypg-635 Keyboard today for the first time to find out that it was not recognized.  It worked fine on my old windows 7 machine. 
 
I looked for a new driver which I think I found the right one UM314x64, but I get an error that says ERROR 25001. No Device is connected.  But, the keyboard is connected.  I tried disconnecting it, and then reconnecting it, but the computer does not recognize it.  Maybe the wrong driver or I am not doing it the correct way.
 
Can anyone please instruct me on how to get it to work on windows 8.1
 
thanks

Try this:
 
Power off the keyboard.
 
Uninstall the driver.
 
Make sure you have the correct driver for Windows 8.1 (not 8) and 64 bit.  Install it.  Reboot.
 
Power on the keyboard.

http://download.yamaha.com/search/detail/?site=usa.yamaha.com&language=en&category_id1=16277&category_id2=16084&product_id=792251&asset_file_language=EN&asset_id=65807
 
note there is  also intallation PDF file on this page, the drivers for your keyboard are good for Windows 7 thru 10.  

Not sure if this is applicable or not, but some hardware devices do not take kindly to USB 3 ports or USB hubs.

Hello,
I've gotten the same error when trying to install the drivers.  I called Yamaha customer service, and the gentleman I spoke with told me to try another computer / cable, and if neither of those combinations work to resolve the issue, the USB connector inside the piano has likely come loose.  He said that if I'm handy, I could open up the front panel and check it out.  On Thursday night, I plan to remove the board, remove the connector, and solder a new one in place.  Below is a Youtube video of an identical repair being performed - it is pretty simple.  

I am hopeful about this fix, and I will let you know if I have success.  My deepest sympathies by the way - all I want right now is for this to work, and that error message is extremely frustrating.  
Regards,
MattG