Thank you all for your input!
Forcing the standard FTDI USB serial port driver onto the "Amicus serial port" does "work" in that it shows up as a COM-port (instead of under "other devices") in the device manager but obviously the Amicus Loader application isn't looking for a "generic" COM port, it's looking for the "Amicus serial port" - which sucks.

In the Proton forum thread I linked to previously there appears to be a special version of the Amicus loader application (in post #8). I've registred and got my account activated but I'm not allowed to post or download so I can't try it and it doesn't really say what the purpose of that version is.

I guess since Windows now sees the board as a normal COM port using FT_Prog to get rid of the Amicus stuff all together is the best thing to do and the way forward.

Dave, are you using the standard MCSPX bootloader or the "UMC" one. I can't seem to find a prebuilt loader .hex file for either 25K20 or the 25/26K22 and I've never used the UMC one.

/Henrik.