so, in every syslog from the past, i found this line:
Sep 11 22:26:02 Unraid-1 crond[1944]: exit status 255 from user root php /usr/local/emhttp/plugins/community.applications/scripts/notices.php > /dev/null 2>&1
after that, the GUI stopped working.
i unistalled and reinstalled the plugin. now the unraid gui workes fine for the last 3 weeks.
i think there was a problem with the "User Notes" Plugin: