APKs are now signed and have changed their appId. This means it will install on your phone as if it were a different app thus you will lose all local data (mainly added / logged in instances). This change was necessary to prepare for Play Store release.
Wait, it is on fdroid? We have not finished releasing it on fdroid, would you mind sending me a link to it?
hmm don’t know if you still need it, but here https://f-droid.org/packages/com.krawieck.lemmur/
it looks like it’s on the Izzy Repository