We have been struggling with databases locking. It appears that the nserver.exe and cluster replicator both have a handle to the database at the same time.
The only way to resolve this is to restart the server. Afterwards, the database that was locked if corrupt. Fixup, compact, etc. will not correct the corruption.
Database Fixup: Unable to fixup database F:\Domino\data\xxx.nsf: Invalid or nonexistent document