Some files are not syncing/accessible anymore

Dear community, I’m facing problems with a nextcloud-instance using docker-containers on a virtual server after running smoothly for more than a year with several version-upgrades during this time. It’s my first post here :wink:

Nextcloud version : 27.1.2 running in most recent docker environment
Operating system and version : Debian 10
The sync-client (Nextcloud on PC/Win10)

The issue you are facing:
Problem: Nextcloud cloud instance is not syncing some files anymore, these files are also not accessible via the Web-Interface either, what the other files are. All of the files are inside shared folders (One person has shared, 3 other group-members have write-access)

Is this the first time you’ve seen this error? (Y/N): Yes and it effects probably 10 - 20 files of several hundreds within different shared folders.

Steps to replicate it:

  1. Simply start a sync on any of the PCs of the group-members
  2. Or try to open the file via the webinterface, other files inside the same folder work
  3. It is also not possible to open older versions from the version-history

The output of your Nextcloud log in Admin > Logging:

Sync-Client: "500 Internal server error" on "GET https://nextcloud....." 

What can I do to dig deeper into this?
Where is the apache-log when using a docker installation?

Help and hints are very appreciated,