I'm not sure whether it's related or not, but it sounds like it could be in the same ballpark. I was unable to run the SYXG50 software synth along with my USB drivers for the Motif. I got a lot of error messages, mostly to the effect of " you must uninstall the previous Yamaha drivers before installing the SYXG50", or "you already have a Yamaha synth connected to your computer".
No matter what I did, the two would not coexist on my computer. I even tried removing the Usb drivers, but it still would not work. I was only able to install the SYXG50 after I rolled back the system to a date before I had installed the usb drivers. Then, once the SYXG50 was installed... same story. I couldn't install the USb drivers. At a certain point, I just gave up. I need the Usb drivers more than I need the SYXG50, so the SYXG50 is gone.
I was tempted to contact Yamaha support, but I just lost interest. Still, this might be where you need to go for an answer. I am running Windows XP btw, if it matters.
AJ
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AJ