Subject: Database is currently in use by you or another user
Obviously, the log.nsf file is ALWAYS in use if a server is running, so the message is correct. The only way to compact those databases (names.nsf is another one) is to quit the server and run ncompact.exe on the log.nsf file.
Subject: RE: Database is currently in use by you or another user
Using the admin client, the compact dialog includes a couple of check boxes… one of which is to allow access while compacting. I’m not sure if that would work, but its worth a shot.