MSIR.0821 File in use for file/group objects, command rejected
A Model 204 command which would modify certain important file structures was rejected because there were file or group objects that referenced the file. This is almost certainly caused by global or session file or group objects. The global objects can be cleaned up by bumping the users holding the global objects -- SirMon should at least make it possible to determine which user(s) have the file in question open.
Session objects are more complicated. First, it should be determined whether the session PST has the file open. If so, action should be taken to clean up the objects associated with the problem file. Unfortunately, there's no really good way to determine which sessions have objects for which files so there's probably not much better to do than to start deleting sessions until the condition clears up.