First, set your Restore Interval to None. I have seen this problem when a Restore interval is set to some period
If that is of no help, check to see if Protection on your backup is still in place. In the Internal Database context, expand sessions, pick a session ID that shows Completed, Click the |+| sign next to the Session ID, to expand it, right-click on one of the Objects listed, and pick Properties, one of the lines will show the Protection
Or, from the command prompt on the cell manager
omnidb -session [session_id] -detail Do not include the [ ] marks
The symptoms you describe indicate that there is no Catalog in the IDB for this backup. The catalog is removed, if the session is not protected, every day at noon my an automatic Maintenance procedure