"Error 422: Validation has failed" when publishing to musescore.com

Im using m4 and want to publish my score to the site.

Initially, the program promts me to login, and this succeeds.
Then, M4 alerts me that it needs to generate mp3.
When I confirm, it takes a bit of time, and then the message appears:

Error 422: Validation has failed.

I retried. 2nd time, I am not promted to sign in (appearently, the first sign in worked).
But it does re-generate the mp3 again, even though that hasn't changed.
Error occurs again.

What's up?