barrier 2.4.0 won't connect to servers as long as a SHA-256 fingerprint hash of the server is missing in the trusted servers data base. Older barrier setups would have installed SHA-1 fingerprints only.
In case anyone else is bitten by this, extract the SHA256 fingerprint on the barrier server:
cd $HOME/.local/share/barrier/SSL umask 077 openssl x509 -in Barrier.pem -fingerprint -sha256 -noout | sed s/.*=/v2:sha256:/ >Fingerprints/Local.txt
... and add the contents of 'Local.txt' to $HOME/.local/share/barrier/SSL/Fingerprints/TrustedServers.txt on clients.
I suppose the same can be done with the UI tool "barrier" somehow, but at least on my Leap 15.3 server I haven't figured it out.