@james huff. Thank you. I have no Development Mode defined anywhere. I added apply_filters(‘jetpack_offline_mode’, true);
in my function.php and still: my debug.log ist filled with
[19-Aug-2020 17:10:34 UTC] PHP Deprecated: jetpack_development_mode ist seit Version 8.8.0 veraltet! Benutze stattdessen jetpack_offline_mode. in /mnt/web408/a3/59/52854259/htdocs/WordPress_04/wp-includes/functions.php on line 5148
This happend after last Jetpack-update.