Well, everything is OK now. After ispconfig finished the /tmp fs was full. After I deleted the backup files it had tons of space in there. By default my comptuer allocates only 16M to the temp fs, so I changed it in the fstab to 64 and rebooted.
I was just pointing this problem out as something that may be good to address in future versions. Maybe check the return value of the process that creates the backup and if the backup doesn't work, prompt the user for a new location or to proceed without a backup.