from oracle documentation
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ORA-00236 snapshot operation disallowed: mounted controlfile is a backup

Cause: Attempting to invoke cfileSetSnapshotName, cfileMakeAndUseSnapshot, or cfileUseSnapshot when the currently mounted control file is a backup control file.

Action: Mount a current control file and retry the operation.
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as for your suggestion, the control file is already there.

what do you think could be the problem?

Regards.