I was affected by this too, but I’m not using encryption:

# sudo -u www-data php occ encryption:status
  - enabled: false
  - defaultModule: OC_DEFAULT_MODULE

The server log just showed ServerNotAvailableException:"Could not decrypt key" for any connection attempts after 19->20 upgrade.

I have another instance where I am using encryption, and there I was able to log in just fine. I did some diffing and found the working installation (with encryption enabled) had 'encryption.legacy_format_support' => true,, while the non-working installation (with encryption disabled) didn’t. Adding that to my config.php alone didn’t fix the issue, so I also added 'encryption.key_storage_migrated' => false, and after that I was able to log in again.

According to documentation, starting with version 20 the legacy encryption is off by default. occ encryption:scan:legacy-format is supposed to tell whether you can disable the compatibility mode, but for the instance without encryption it seems to produce a false positive (complete with a typo):

All scanned files are propperly encrypted. You can disable the legacy compatibility mode.

For the admin user, the now otherwise working, unencrypted instance also shows a ”Please enable server side encryption in the admin settings in order to use the encryption module” notification in the web UI, for every page load (i.e. even after dismissal it just pops back up whenever I navigate in the UI).

The errors reported by app:check-code encryption are probably unrelated, as the same errors are reported for both my instances.