I have had the same issue. I have just resolved it by creating a new submit
data connection and using that instead. It didn't appear to work at first,
but any new forms created after modifying the submit method can now be
successfully overwritten.

During my attempts to resolve the issue, I removed the tick to allow forms
to be overwritted, then re-instated it after. I haven't checked to see if
that would fix it on its own. I am just glad it now works.

Good luck. I hope this helps.