The submitted information is added to my entries. However, I do not receive any notification email. And, the biggest issue is ofc the error message everybody gets after entering their information.
Do anybody have an idea of what causes this issue?
That critical error should trigger an email sent to the site administrator. That message will contain information about the actual error, which is critical to resolving the issue. If you do not receive that email, you can check the web server error log or PHP error log, or, follow the link to learn more about debugging in WordPress:
I checked my plugins for conflicts and it happened to be either the Gravity Forms Signature Add-on by ApproveMe plugin or the Gravity Forms User Registration Add-On plugin that was causing the problem.