I think it has something to do with the number of files. Just encrypting / decrypting a 10GB random data file did not show a problem.

It is propably related to decrypting large (single) tar-files. It works flawlessly when renaming the tar-files to another extension before encrypting and afterwards decrypting it again. But as long as it is named xyz.tar Kleopatra crashes. Could it be that untarring causes some "out of memory" failure? I recognized that while decrypting the tar there was no sign that the decryption process would allocate any disk space. There is just an empty randomly named folder being created upon decryption.

I am pretty sure that this was related to issues we found when analyzing another crash / hang with Kleopatra. In T5478 we are currently reworking how we handle archives completely. This will fix this issue, too.