What does it mean "form over quota"

The error message "Form Over Quota" means that the specific form has either reached its limit as a form or that the account has reached its limit, so the forms got disabled because of it.

Seeing that you are on our Silver plan, your account is not near any limits, so the most likely reason why you would see a message like that would be if the specific form has reached its own limits - which are set by you.

You can see more about Form Limits here: How to Limit Submissions in a Form

If you have some form that has this limit, do let us know and we would be happy to assist if needed :)