At the moment Matomo only shows the following error:

Fehler : Sicherheitschecks fehlgeschlagen. Bitte laden Sie das Formular erneut und prüfen Sie, ob Ihr Browser Cookies zulässt. Wenn Sie einen Proxy Server verwenden, müssen Sie Matomo so einrichten, dass es Proxy Header akzeptiert.

But when the user has Cookies enabled (which is pretty likely) and is sure that they don't use a reverse proxy (shouldn't it say reverse proxy instead of proxy in the message?), there is no way for them to troubleshoot this issue further, and they will most likely just give up on using Matomo.

Maybe all checks that could fail in verifyNonce() and isLocalurl("") should be logged or even help display a more helpful error message.