I rewrote the content template completely and compared it with the first template.
And I found the reason:
One customfield had been modified, instead of its former type "checkbox" it was changed "radio-field"
To find the reason for the white screen took me a long long time.
Debug did not show anything.
Any error-message if a customfieldtype has a wrong output definition in the template would be very very helpful
An errormessage is 100times better than a white screen
Thank you for that!