Inconsistent Backup failure

I am facing an inconsistant behavior that I cannot explain:

Created a new directory inside my drupal directory, ContentBackup1, and entered it in the module setup as the "Backup location".

"Capture errors from backup run" is Unchecked
"Backup from parent directory" is unchecked.

Some times I get en error: "backup_files(): Command 'tar cfz /tmp/backup-htdocs-xyxyx ... (list of the files and directories) ... (the sql.gz file) returned value: 2"

Other times (I create a different directory, as a test) the backup is completed without an error. A ...tar.gz file, owned by apache/apache, mode 644, is created in the backup directory.

System is Centos
I am accessing it via Firefox on Mac (to avoid the current problem with Safari).
apache is a member of the group that own the backup directory that has mode 775.

Any idea what to check???