Did you start with a new database or did you re-use the database from 8.2.6? If you re-used the same db connection, I think you will need to run the “occ upgrade” command (which you may not be able to do as you’ve mentioned you have no shell access"
Have a look at https://help.nextcloud.com/t/problem-with-code-integrity-after-uprade/1156. This has been reported and I believe there are a couple of issues open on github around this:
Another thing to look at is the new updater script here:
https://help.nextcloud.com/t/test-our-new-work-in-progress-upgrader-script/1214