Thanks chewitt. I already asked this on github as well. but:
problem solved.
go to settings and chose "user specified" when asked for a vpn service. (not ExpressVPN)
enter your credentials and keys.
start the vpn and you will receive an error log prompting you to make slight changes in the .ovpn-file:
delete "ns-cert-type server" and add "remote-cert-tls server" instead.
That solved the prob for me