Not using on RHEL5 - but on CentOS 5.3 which should be the same thing.
Both Full and Diff work correctly.
Did you copy above directly from console ?
Diff Backup started at: 09:21
archive enable = no
Doing a hotsync of to
nice: a command must be given with an adjustment
The fact that you have no folders listed in the Doing a hotsync line above suggests you have not run the --INSTALL option to set up the correct folders for the backup.
Here is what I get when I run ...
"Doing a hotsync of /opt/zimbra/ to /tmp/fakebackup/"
PM me with an email address, and I can send on my tweaked copy of the script if that is of use to you.