Regarding: "The overview information in the backend complains only that "Locale setting has failed", no spam filter and "no mail handler configured"."
This was just mentioned to say, that the server settings should be fine.
Regarding the problems why I opened this topic:
PHP-Error.log: I have no clue where to find it within DebianEdu.
"normal" error.log has following entries:
[Mon Jan 15 16:26:45.050980 2018] [:error] [pid 4327] [client ::1:56466] script '/etc/debian-edu/www/zenphoto/zp-core/zp-extensions/zenpage/admin-upload.php' not found or unable to stat, referer: http://localhost/zenphoto/zp-core/zp-extensions/zenpage/admin-news-articles.php?publish=1&titlelink=erste-gschichte&XSRFToken=cd3b93f3501aa85ac16674963560117eb37a4762
and
[Mon Jan 15 16:26:58.056133 2018] [:error] [pid 35151] [client ::1:56468] script '/etc/debian-edu/www/zenphoto/zp-core/zp-extensions/menu_manager/admin-upload.php' not found or unable to stat, referer: http://localhost/zenphoto/zp-core/zp-extensions/menu_manager/menu_tab.php?page=menu&tab=menu