Well I'll be damned, should have thought about this myself
First:
thank you for your help! Greatly appreciated!
I already had excluded the drive with MB (and the FabFilter .dll) from Windows Defender so initially I thought that could not be the problem. But here's the thing: the problem always occurred in the first minutes after booting my PC (that's also the time frame I always start MB).
So... this time I disabled Defender right after booting and after that I opened MB: no problem! Rebooted four times in a row using this method: all succesful!
My theory: the Defender exclusions only take effect
several minutes after Windows has booted.
What's weird is that Defender doesn't see the .dll as a virus but for some reason it
does interfere with MB loading it...
I don't have a real solution yet (e.g. some kind of Powershell script to delay Windows Defender at Windows boot).
For now I have to do this manually I guess... Oh, well...