Edit the user and when the form is returned with an "invisible" error, then please scroll down to the "Powered by WoltLab Suite" text at the very bottom. Right click on it and click on "Inspect element", this fires up the browser's developer tools.
The button itself doesn't matter, but what you should look for is this block:
This is an emergency fallback for badly behaving input validation that gives a hint on what is happening. This is always added to the document source whenever a form was rejected.