I finally discovered what made the scanner crash, and it's stupendously simple.

First of all, I should say that I only have local films in my library, and that all of them have their own separate .nfo file with the title and other info like IMDB rating etc.

As it turns out, the scanner has a problem with some diacritic letters, which makes it just stop scanning and output "An error occurred"-message. I realized this when the scanner did not crash when it had half my library loaded, and after some searching, I managed to narrow it down to a French film with the word "intérieurs" in the title. As you can see, the title has a tiny ' above the e in "intérieurs", and that was enough to make the missing movie scanner crash.  So I just edited the .nfo for this movie and set the title to be "interieurs" (without the ' above the e) and suddenly the scanner worked perfectly and I could finally scan my whole library again.

I have a whole bunch of other films which has other diacritic letters in their title, so it seems the scanner does not have a problem with all these letters, but at least é was a problem.