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.
The latest build (0.2.0) has a log out button next to the account name (Profile > Settings > Account).
On older versions, I think you have to log in to more than one account, or one account twice, before the option to log out appears. If one account is logged in twice, it will fully log out.