Modify Diagnostic yields Internal Server Error

After modifying an existing Diagnostic in its field "Details" and clicking submit, the URL is "https://fwkshotsheet1.fz-rossendorf.de/handle_form_submission" and the browser displays

Internal Server Error; The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.

Yet, the change in the field is propagated. One can restore operating the app by returning to the URL "https://fwkshotsheet1.fz-rossendorf.de"