Get error "App not installed" on Lineage OS 15.1 with MicroG when trying to install an Apk file

[AnotherElk]

Well, if nobody else comes up with something I have an answer … but a rather unpopular one, I believe …

@AnotherElk

My personal opinion is that in general it’s better to install the phone from scratch when upgrading the major Android version, and I know it means investing time and work.
But if something doesn’t work out, there’s one less question to ask … Could just upgrading the existing installation instead of installing from scratch have caused the problem? (Answer to this mostly is: Yes, making troubleshooting more complicated.)