During diagnosis of a different problem, I was presented with an on-screen (console) message reading, in part...
sh: -c: line 0: syntax error near unexpected token `('
sh: -c: line 0: `mkisofs -no-emul-boot -b isolinux.bin -boot-load-size
4 -boot-info-table -c boot.cat -o /backup//Mondo- (test)
-Birch-061025@2031-1.iso -J -r -p MondoRescue? -publisher
www.mondorescue.org -A Mondo_Rescue_GPL -V 1 . >>
/var/log/mondo-archive.log 2>> /var/log/mondo-archive.log'
Call to mkisofs to make ISO (ISO #1) ...failed
---promptdialogYN---1--- Failed to burn ISO #1. Retry?
However, these error messages didn't appear in the log. When I submitted the information to the mondo-devel list, I included both the (compressed) lot, and this snapshot of the screen (above).
It might be useful to make sure all error messages are echoed to the log, so sending you that log would be adequate for your further investigation.