The only reason I could find for this is that part of the IDB is missing, in this case the Media Managemetn Database
You don't state the OS-type of your cell manager, but, at a time when no backups are running, run these commands:
Windows Cell Manager
cd \Program Files\Omniback\bin
omnidbcheck -extended
omnidbutil -info
omnidbutil -extendinfo
UNIX Cell Manager
cd /opt/omni/sbin
./omnidbcheck -extended
./omnidbutil -info
./omnidbutil -extendinfo
You should know, physically, where the last good IDB backup is located, because, I see the definate possibility that you may need to start with a new IDB