Notice that a file that has an exclusive lock (ie already opened in exclusive mode for ex) will fail to be opened again if the lock hasn't been released correctly.

If you had a crash and a pending lock on the file, better reboot... (or try to get ride of the lock by calling the UnLock/Close whatever is holding the lock on the file if you still have the related BPTR. Nothing impossible but a reboot might be better in the end...).