gnome3 desktop application show up in activities overview but not in default applications

I have two identical systems running both debian bullseye with gnome3 (3.38.5) and I wanted to add bettebird as my default application for mails. I've created a desktop file in ~/.local/share/applications on system 1 and everything works fine on system 1 - I can select betterbird as the default mail program... I've copied this file to system 2 and betterbird does show up in the activities overview, however, in the 'default application' settings it's not visible. I'm really puzzled & running out of ideas how I can fix this. What I checked/tried after googling for a long time but no success: - Icon & Exec are correct (I can start Betterbird from activities menu) - I have other custom desktop files in ~/.local/share/applications/ and they do work - I changed rights from 644 to 755 (though other desktop files in ~/.local/share/applications do have 644 and do work...) -> no luck of course - I copied thunderbird.desktop from /usr/share/applications to my local folder & adapted entries/renamed file accordingly -> no luck - I tried to open a file with betterbird from my file explorer (nautilus) but Betterbird is also not available from the application list Any ideas how to proceed?