After the hack last night, effected instances, lemmy.world being one of them, invalidated existing login tokens as a security measure after updating. If it is persisting after deleting the app, you may want to try my personal fix. What I did was

Log into alt account in addition to my primary one
After you have 2 accounts added to the app the option to delete, basically log out of the account, will appear by both accounts (memmy only lets you do this if there is more than one account, it won't let you sign out if it is the only account on the app) then I deleted the primary account from the app.
I exit the app and close the tab
Open the app again, and resign into primary account by hitting the add account option.

[–] SomeoneElse@sh.itjust.works 4 points 1 year ago

I did this earlier today and it did the trick. But something is up with lemmy.world again now and that fox isn’t working. I’ve had to switch to a different instance and now Memmy works fine.

[–] TheGoldenGod@lemmy.world 2 points 1 year ago

Already did that, but if you look at my earlier post, they acknowledged Lemmy.world is having issues after I posted this. Left it up incase anyone else came in with the same question.