Many thanks for your help!
In the meantime I found out that it is probably due to the plugin “Classic Editor”. If I deactivate the Classic Editor and work with Gutenberg, error messages will no longer appear. This is a big problem for me because I work with HTML5 / CSS3 and do not use Gutenberg.
In addition, a new problem has emerged today. When updating to the offered version 3.8.1 I receive the error message:
Undefined index: package in /homepages/28/d642563536/htdocs/eichholz/wp-admin/includes/class-theme-upgrader.php on line 374
An error occurred while updating Parallax Frame Pro: update package not available.
Thank you for your effort.