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ORA-01623:

log string is current log for instance string (thread string) - cannot drop
Cause: A thread's current log cannot be dropped even if the thread is closed. A disabled thread usually does not have a current log, but a half completed disable may need to be disabled again.
Action: If the database is not open then disable the thread. If the database is open and an instance has the thread open, then the instance can be requested to switch logs. If the database is closed the log can be archived or cleared to force a switch.