Given what you know now, do you have any guess as to what the problem was (and/or could still be) if it wasn't file size limit?
Sorry, not really. I have experienced the problem before, but it was during the operation of a db (not mine). If you research the issue as Orange suggests (here or elsewhere) there's a lot of posting about it, with the primary 'fix' being a registry tweak. The usual suspects don't seem to apply in the case of doing c/r on a db because you're not running any queries or code.
I suggest you look into Task Scheduler to perform your backups or create a recurring Outlook appointment or task so that you don't forget. TS may not be viable in your case.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.