The popup was about the database and was displayed shortly after the beta installation was completed. As you pointed out, it was caused by uninstalling the stable version. I installed 2.1 again and finally reached the beta version. There is no problem with the Microsoft Store with this version. Thanks for your advice!What displayed this message? TinyWall's installer? Or TinyWall itself? Or something completely else? The only similar message in TinyWall is "Database is missing or corrupt.", but that seems to be a different message (or maybe the same but original meaning lost in translation?).
You can/should just install the beta while 2.1 is installed. It will automatically upgrade the version AND migrate your firewall settings. It is unnecessary to uninstall the older version first.
Downgrading is a different story: To downgrade, you need to uninstall the newer version first and then install the old version.
It is very important in general, that you only use the installer and uninstaller to install, remove and upgrade. Never try to upgrade or remove the files manually without using the (un)installer. This is of course true for all other security software too that have non-trivial integration with the system, but I need to point out TinyWall is no exception. You haven't done any manual file deletions or upgrades have you?