It means something is wrong with the file you are trying to import.
That can not be. Otherwise I would not have a problem with the old FBX files, but the old files DO NOT work now!
Since the old FBX files all worked early (today nothing more) and I have my models easily imported, I assume that it is due to importers.
Or have the old file also called for a hunger strike ... probably from lying too long without being touched ...
I've deleted
eng.log (previously it helped) but no result ... it's ERROR time and time again