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.
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