Please make sure that nx-libs 3.5.99.16 is installed on the server side.
This sounds like a typical incarnation of the DPMS bug, fixed with nx-libs 3.5.99.15.
However, this doesn't sound like it. Normally the DPMS bug only manifests after a pretty long time (say, an hour or 1.5h, depending on DPMS settings), but not right away after starting the session. I haven't yet seen such an error message yet. It's possible that it's caused by the session repositioning on non-first displays/nxagent crashing bug as fixed with nx-libs 3.5.99.16, but normally that incarnates differently (i.e., nxagent is being started successfully, but then dies quickly while starting the session without a client-side error message).
NXAGENT - Version 3.5.99.16
Copyright (c) 2001, 2011 NoMachine
(http://www.nomachine.com)
....
See https://github.com/ArcticaProject/nx-libs for more
information.
Info: Agent running with pid '17844'.
Session: Starting session at 'Mon Mar 26 16:56:11
2018'.
Error: Aborting session with 'Unable to open display
'nx/nx,options=/tmp/.x2go-fangq/C-fangq-50-1522097770_stDXFCE_dp24/options:50''.
Session: Aborting session at 'Mon Mar 26 16:56:11
2018'.
Session: Session aborted at 'Mon Mar 26 16:56:11 2018'.
"PANIC!" Failure negotiating the session in stage 7
...
Wrong version or invalid session authentication cookie.
....
Mihai