Welcome to the Communtiy page,
Can you turn your mtd off and then on again and try to submit your return.
Cog>account and settings>advanced> HMRC> disconnect and then reconnect it> you will need to reput in your government gateway id and password in so make sure you have them to hand before you disconnect it. once you have followed the steps to reconnect, go back to taxes and try to resubmit the return.
Let us know how you get on.