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.
Can confirm removing and re-adding fixed my issues with multiple accounts across instances. Had trouble upvoting, seeing subscriptions, etc. Now all is fixed with all accounts in Memmy.