I have converted resources on some turns where this occurred, but only the excess above what was needed for current construction, maintenance, and retros. If this were the problem, I would expect that at least 1 or 2 ships to be retro'ed before getting the lack of resources message.
This Last turn, I did not convert any resources, so that shouldn't have been a problem. I did accept a gift of a small quantity of resources, but that didn't significantly affect my totals.
Thanks for the hints. I'm doing so poorly that even if it works now it may be too late (but I hope not
)...Greybeard