I did some further research:
Whatever folder I enter behind our Nextcloud domainname, for example https://cloud.domain.com/bla it is always redirecting to https://cloud.domain.com/apps/files/?dir=/&fileid=893 so the files seems to be the default folder where it redirects if the requested URL isn’t found.
The following volumes are connected into the docker container:
volumes:
- ./data:/var/www/html
- ./app/config:/var/www/html/config
- ./app/custom_apps:/var/www/html/custom_apps
- ./app/data:/var/www/html/data
- ./app/themes:/var/www/html/themes
If I have a look into ./app/ folder should I find anything having to do with the updater in some of the sub-folders? Maybe something is missing there?
//Edit: I found the requested folder (first line in the log above) /apps/updatenotification/ but not /apps/updatenotification/credentials. Could this be the cause? If yes, any ideas how to resolve without reinstalling?