I wouldn’t suggest downgrading, because the database schemes might no longer be fitting. So it’s a bit… well… risky.
Safest way to downgrade is always having a snapshot / backup.
Anyway, I can’t reproduce this issue.
Please try cleaning your cache and reloading the login page.
If the issue persists, run
/opt/zammad/contrib/packager.io/postinstall.sh just for safety.
Again, reload the WebApp.
If it still does not work, let me know. This normally should fix it, if it appears.