More info:
On the Proton forum I found a thread with links showing how to install unsigned driver so I followed that and was able to install the Amicus 18 serial port driver (which again is nothing other than the standard FTDI driver as far as I understand) and it shows up correctly in device manager. But when rebooting into normal mode and plugging in the Amicus 18 it again refuses to use the driver.

For reference, here's the thread on the Proton forum.

/Henrik.