This is still occasionally happening. Have upgraded at least a couple of times since my original post.
However, I might have found that it is linked to creating an invoice from the delivery window. When you create an invoice from a delivery it opens up the invoice window but it doesn't seem to save the record. Although it retrieves next number.
If my customer ticks the ok box and presses save rather than saving first before oking we seem to get this error.
Anything anyone can add would be appreciated
Thanks
Lee