Bernhard,
To add to what Allison said, the Error Trace shows errors that we log when trying to sync email or form data to your CRM. In this case, it sounds like the form post is not successful, so we are not getting the data to try to sync to your CRM, thus there is nothing logged in the Error Trace. The URL on the error page for a form capture will contain more detail and our support team can help troubleshoot it if the detail is not sufficiently helpful to you.