The usbser.sys error in the management is now just a warning thanks to the disabled driver signature verification. But the error 39 when installing drivers persist (despite copying usbser.sys directly into the Arduino folder) with my antivirus deactivated.
The error may be related to amd64 drivers. The installer takes a long time to work itself through that task and the sfc reports activity during the verification of those files. Scanning in safe mode, as you advised, at least doesn't end in "problem couldn't be solved".
I ran CCleaner and Wise Registry Cleaner in the Advanced mode after uninstalling.
There are so many things I have done, that I couldn't remenber them all. One of them are the two you advise (deleting infcache and as many devices as possible in the device manager). I had tried them already.
Every try makes it clearer that it is a very specific and time-consuming problem in my computer.
Yes, Vista died long ago... maybe this episode is the last hint I needed to bury it.
If the few new things make you think of another possible try I could do, I would thank you. If not, I very much appreciate your help and time and would invest in a long-term solution.