There seems to be a new development: I reinstalled eM Client again a few days ago, but this time I left the database location the standard way. (I always had the database located at another partition than the C: partition, which has been working flawlessly for quite some time.) I did not have a single problem since, and I tried to provoke it quite a few times. So I don’t know what exactly is making the difference; maybe eM client just likes the standard location better. Or it’s the fact that my C disk is an SSD, and the other partition I used before, is a standard (slower) hard-drive? Whatever it is, it also must have something to do with Windows itself, because the non-standard location was not a problem earlier.
Anyway, I hope it keeps functioning like it does now!