ok, I've localised the problem, literally in this case 😀

If you run dosbox from a different folder then where it is located, in this case, in a batch file like this:
c:\games\dosbox\dosbox -c "imgmount d c:\games\lba1\cdrom\lba1.iso -t iso"

then it crashes. However if I move the batch file into the dosbox folder, it works without crashing. In every other case where there is no ISO is mounted, dosbox works fine. I've tried putting:
cd "c:\games\dosbox"

in the batch file as well, so it was in the folder itself, but it didn't make any difference to my suprise.