Unfortunately another error after using the Web Updater. Attempt to update to Nextcloud 28.0.0 beta 2 via Web Updater, a server error is displayed after the update has finished. Why? I have to restore a backup.
Beta is not suitable for Production. Using Beta version assumes you know what you are doing and can read logs yourself and come to a solution yourself. After that you report back to github that you have found a bug and post your results.
You cannot solve NC v27 errors by upgrading to NC v28 unless stated in the changelog.
The error you got is shown because something went wrong. as you do not supply the error no one can help you or tell you exactly why.
Yes restore your backup.