Exit codes come from operating system problems. The lines before the exit code should provide a clue; from some of the technotes on SupportNet, it could be out of memory, that the LD_LIBRARY_PATH has the wrong version of a library first, or that the license server does not have enough licenses.
Hope that helps-
Hi Samantha, thanks so much for your reply! I checked on Supportnet and found that "code 1" can be due to physical space on disk (https://supportnet.mentor.com/portal?do=reference.technote&id=MG80655&lang=en&prod=C110-S105-G251-P10088), and this was in fact the reason (by installing the new PDK version I almost filled completely the partition where also Calibre runs!). So I freed some space and the message was gone!
It would be very useful if a brief description of the exit codes was included as part of the standard Calibre documentation, though.
Thanks again for your help!