I was editing a block using the "Edit block in-place" and then (because I was exploring QCad a little) tried the "Purge unused blocks" option. I suspect that the purge removed the block I was editing, since when I now click "Save and close", I get "Original block not found". Also when I click "Cancel", I get "Original block reference data not found".

Recreating an empty block with the same name fixes this, and "Save and close" seems to work as expected again.

This is probably very much a corner case which is not really important, but I wanted to report it just so it is known.