The “Internal Server Error” message is shown on a blank page (probably from nginx web-server) or it is formatted as a nextcloud page with a “Technical details section” and a “Request ID” (the request id can be used to search and pinpoint the problem)?
If you can access some nextcloud pages I guess is the latter option.
On the Files app, it shows OK but without listing the files or doesn’t show anything other than the error?
Currently installed Nextcloud version is Nextcloud Hub 6 (27.1.11)?
On NS8 you can see the log for the NS8 nextcloud app instance (unified view of internal modules/services).
On Nextcloud’s Logging section (as admin) you can see nextcloud.log and if more verbose info is needed it is possible to increase log verbosity to debug level (for a short period of time, undoing changes later):
Sometimes you can see other errors on your web-browser console.