After installing KeePassNatMsg plugin into KeePass2 version 2.51, then restarting KeePass2 no errors are given at start.
Installing NativeMessaging Host does not give rrors either. Installing KeePassXC-Browser in Vivaldi, Chrome, Chromium, Edge, Brave is ok too. Same with Firefox and Thunderbird.
Now trying to connect to an open database does nothing. No popup to give a name to the key used for connections. It is just: nothing happens.
Seen with Ubuntu, xUbuntu, lUbuntu, and Debian.
mono is installed in all cases full.