This means the URL is too long. There's a web standard limitation that we have no control over. The URL should be less than 2048 characters.

If your form is meant to update an existing record, then you should send a miniExtensions form editing URL instead. The main feature of our form is that it allows you to edit existing records. It generates a unique ‘editing URL’ for every record in your table. This article guides you step-by-step on how to set this up.