servername localhost:9943 I can't change it

Hi,
I installed DS2.1 on a client and the license is on a server with PP 6.1.6. When I want to change "servername in DS2.1 interface, it says all the time: "Could not connect to server leviathan. Please select another Pipeline Pilot server to connect to."
Could you highlight what is wrong, please ?
I checked the port 9943 and 9944 are open. When I'm typing : lp_server -w, I obtained
server connection: 1715@leviathan
status: 1715@leviathan: Connected
which sounds good.

Thank you in advance,
Best Regards
Nath