The latest Lemmy UI release is not without some bugs. Some of these are new, others aren't, but they're all worth being aware of as they can cause some confusion:
You need to clear your browser cache before you can log out: https://github.com/LemmyNet/lemmy-ui/issues/1792
Note: This is more upgrade related, rather than UI version specific. Logging in and out will work fine once you've cleared your cache.
Lemmy UI still ignores the server listing type preference, which means users that are not logged in will only see Local posts by default: https://github.com/LemmyNet/lemmy-ui/issues/1612 - This one is a bit frustrating for me since we're a small instance and new users may not already know they can click on 'All' for federated content.