Repository Main Settings Save gives error message

Try to save the Main setting of a repository but get error message which differs depending on which browser is used:
Safari: The string did not match the expected pattern
Chrome: Unexpected token < in JSON at position 0
Firefox: JSON.parse: unexpected character at line 1 column 1 of the JSON data

The following URL is sent and gets a forbidden 403:
Request URL:
Request method:PATCH
Remote address:[xxxx:xxxx:x:xxxx:xx::xxxx]:443
Status code: 403
Referrer Policy:no-referrer-when-downgrade

What could be the problem?
On the same settings page the Secrets, Cron Jobs and Badges save is working

Thanks in advance, Lud