Original comment: keepassxreboot/keepassxc-browser#1399 (comment)
Ok reporting back now.
The problem was that the org.keepassxc.keepassxc_browser.json file in my browsers profile directory was removed or not recreated in the most recent update.
It is fixed by creating this file:
/home/USER/.PROFILE_DIRECTORY/native-messaging-hosts/org.keepassxc.keepassxc_browser.json