With me appears in the log file the following entry.
What does that mean ?
Probably something not installed and configured properly. Can you share more information about your setup? Is this on shared hosting? Which webserver do you use?
some more background on these kind of error from our colleagues from the owncloud community:
Does it run a apache webserver for Nextcloud? Most required settings should be handled by the .htaccess but in case of nginx you need to add this to the webserver configuration. Unfortunately, I don’t know the Synology-environment at all, perhaps someone can step in here?