While I can't say for sure what the policy is as I'm new here as well (Hi!), I can say it happened to me when I had a string of submissions - so it isn't user error or something just limited to you. That said, I simply spaced out my submissions by an hour or so. Sometimes I've been able to submit a new one within 15 minutes.

This may be an issue unique to NWC:NES given how many people are running it who aren't traditionally speedrunners and how many levels/categories there are to submit to - I have no idea how the SRC database is set up to receive form data but it could just be some race condition somewhere that isn't usually a problem as 99% of titles don't have dozens/hundreds of submissions a day.