>>If it's 3.5 you will get this error on absolutely accidental basis until you >will upgrade. I had such system, it's MS confirmed problem.
>
>The NT version is 3.51 with SP 5. Thanks for the input.

Anyway, check Server Event Viewer. WinNT Server has bad habit to autoreboot (because of some network interrupt) and at this point your local users will get the error message.