On the 17th of December I wrote/asked in a similar named thread:

> I don't know if this might be related but our customer just told me
> that around the time the errors occur an nbackup is running.
>
> Can nbackup cause such problems?

Now, after the customer switched off the nightly nbackup the problem
seems to be gone.

Are there any knowns issues/bugs with nbackup which I should be aware of?

--Heiko