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.