Re Vincent,
switching off OpLocks may indeed help (AFAIR it means the SMB server claims to not support them, so clients don't use them, so there would be no locking to the backend FS either). But did you check the typical Office situation of a user trying to open a file already in use by another user (or even themselfes, yay terminal services)? If that doesn't recognize the lock, it may wreak havoc sooner or later. ISTR that OpLocks are an additional locking feature above and beyond standard locking and so I might see a problem here were there is none, but I'd test this at least superficially before it goes kaboom on me ;)
Thanks,
Andre.