As others have said this is fixed with signing out and back in. Connect didn't have good handling for this case, where you're signed in successfully but then the server invalidates the credentials. Will be fixed in the future!
I think something is still messed up - I've tried nuking all my lemmy.world cookies and I've done a couple sign-out-sign-in sequences. Those things don't seem to improve things. The way it's acting it's not hard to imagine a messed up load balancer or a split brainy thing somewhere; hitting reload repeatedly will eventually get you a properly rendered page.
Yeah I think something is still messed up too. It gets into the messed up state on Connect when it makes an API call with what it thinks is a valid authorization token but it gets back a "signed out" result. Things should be a bit better on the latest beta version for what it's worth (although API issues are beyond what Connect can do).
I was just having the same problem. Signing out and back in was doing nothing. Right after signing in again I came here and tried to post, and it told me I wasn't signed in. Then I signed it and back in again, and my frontpage finally came out.
I don't know what's going on, but it was like it's telling me I logged in but it didn't actually log me in most of the times I tried.