I've run into an issue updating NextCloud from version 12.0.1 to 12.0.3. The quick version is that somewhere along the way NextCloud downloaded 12.0.2, but it was not used to update. When the updater saw a file in the downloaded updates folder, it was unable to verify the signature.
The fix is to delete the old 12.0.2 zip, download the 12.0.3 zip to that location, then re-run the updater. After that switch, everything went smoothly.