I'm not sure whether to thank you or not. Although the backend problem is fixed, the frontend now shows errors "Deprecated: json_decode(): Passing null to parameter #1 ($json) of type string is deprecated in /home/harmoni2/nieuw.harmoniesimpelveld.nl/plugins/system/helix3/core/classes/menu.php on line 290" which were not there yesterday. Can you please restore whatever you did to make those errors disappear?
Edit: you apparently just enabled all error reporting which was on a lower level before. Excuse me and thank you for the repairs!