Lemmy.world was experiencing authentication issues yesterday, making it difficult to comment and vote.
These issues were resolved some hours ago when lemmy.world updated to 18.02, but you may still be experiencing issues with your account on third party apps such as memmy.
If so, everything will start working normally if you remove your account from the app and add it again! Possibly just reinstalling will also achieve the same thing, but I haven’t tested that personally.
If you go to your profile tab, click settings > accounts > edit account, delete your password, and type in anything other than you password. Try to save this, and it will fail to log you in. Now renter your real password and it will save your account info. This acts as a “refresh” for you account token.
It’s a bit cumbersome, but there’s currently no way to manage accounts with only one logged in.
EDIT: I stand corrected, the latest TestFlight has added a logout button to the account settings page.