Hi! I answered this question on Patreon and on Discord, so I will answer it here, too. There was something changed on the internal part of the Unity Android compiler that caused this frequent error on some devices. I will try to migrate the game to a newer version of Unity for the next release and check if it can fix this problem.
The apk file doesn't have a certificate or the certificate is corrupted it doesn't have anything to do with unity also if you want to fix it use the same certificate than the older version and the demo update will fail otherwise
https://developer.android.com/studio/publish/app-signing.html
https://source.android.com/docs/security/features/apksigning/v2