Hey,
 
I do have the same problem as well. I already tried to delete alle sessions in /tmp/.x2go or HOME/user/.x2go, tried to reinstall, update to latest version, used several client versions, tried with a fresh user, deleted .Xauthority, blocked a working cookie. None of it worked.
Regular ssh -X starting GUI applications work. Can you please help.
 
server side:
x2godesktopsharing: 3.2.0.0
x2goserver: 4.1.0.3
x2goserver-common: 4.1.0.3
x2goserver-desktopsharing: 3.2.0.0
x2goserver-extensions: 4.1.0.3
x2goserver-fmbindings: 4.1.0.3
x2goserver-printing: 4.1.0.3
x2goserver-x2goagent: 3.5.99.19
x2goserver-xsession: 4.1.0.3
 
client side:
4.0.5.2
 
error server side:
Info: Agent running with pid '20056'.
Session: Starting session at 'Thu Apr  4 21:21:43 2019'.
Info: Proxy running in client mode with pid '20056'.
Info: Using errors file '/tmp/.x2go-jlocher/C-jlocher-50-1554405703_stDXFCE_dp24/session.log'.
Info: Using stats file '/tmp/.x2go-jlocher/C-jlocher-50-1554405703_stDXFCE_dp24/C-jlocher-50-1554405703_stDXFCE_dp24/stats'.
Loop: WARNING! Unrecognized session type 'unix-kde-depth_24'. Assuming agent session.
Warning: Unrecognized session type 'unix-kde-depth_24'. Assuming agent session.
Info: Waiting for connection from 'localhost' on socket 'tcp:*:42847'.
Info: Aborting the procedure due to signal '1'.
Error: Aborting session with 'Unable to open display 'nx/nx,options=/tmp/.x2go-jlocher/C-jlocher-50-1554405703_stDXFCE_dp24/options:50''.
Session: Aborting session at 'Thu Apr  4 21:22:43 2019'.
Session: Session aborted at 'Thu Apr  4 21:22:43 2019'.
 
errors client side:
x2go-DEBUG-../src/sshmasterconnection.cpp:1703> "channel_write failed." - "Remote channel is closed"
x2go-DEBUG-../src/sshprocess.cpp:463> I/O error: "channel_write failed."" - Remote channel is closed" (3).
x2go-DEBUG-../src/sshmasterconnection.cpp:1746> EOF sent.
x2go-DEBUG-../src/sshmasterconnection.cpp:1750> Channel closed.
x2go-DEBUG-../src/onmainwindow.cpp:6014> Proxy wrote on stderr: "Error: The remote NX proxy closed the connection.
"
x2go-DEBUG-../src/onmainwindow.cpp:6014> Proxy wrote on stderr: "Error: Failure negotiating the session in stage '7'.
Error: Wrong version or invalid session authentication cookie.
Session: Terminating session at 'Fri Apr  5 14:49:45 2019'.
Session: Session terminated at 'Fri Apr  5 14:49:45 2019'.
"
x2go-DEBUG-../src/onmainwindow.cpp:5871> Deleting Proxy.
x2go-DEBUG-../src/onmainwindow.cpp:5920> Waiting for proxy to exit.
x2go-DEBUG-../src/onmainwindow.cpp:5940> Checking exit status.
x2go-DEBUG-../src/sshprocess.cpp:199> Executing remote command via SshProcess object 4: "x2gocmdexitmessage test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/sshprocess.cpp:204> this=SshProcess(0x55c0a89e3360)  Running masterCon->addChannelConnection(this, ' "7c6b18fd-106d-46c1-8243-477c4f699384" ', ' "bash -l -c 'echo "X2GODATABEGIN:7c6b18fd-106d-46c1-8243-477c4f699384"; export PATH="/usr/local/bin:/usr/bin:/bin"; export TERM="dumb"; x2gocmdexitmessage test-50-1554468584_stDXFCE_dp24; echo "X2GODAT" ');
x2go-DEBUG-../src/sshmasterconnection.cpp:1324> Locking SSH channel connection MUTEX.
x2go-DEBUG-../src/sshmasterconnection.cpp:1326> Passing new channel conenction object to channelConnections.
x2go-DEBUG-../src/sshmasterconnection.cpp:1328> Unlocking SSH channel connection MUTEX.
x2go-DEBUG-../src/sshmasterconnection.cpp:1516> Creating new channel.
x2go-DEBUG-../src/sshmasterconnection.cpp:1520> New channel:0x7f54e00113f0
x2go-DEBUG-../src/sshmasterconnection.cpp:1551> Executing remote: "bash -l -c 'echo "X2GODATABEGIN:7c6b18fd-106d-46c1-8243-477c4f699384"; export PATH="/usr/local/bin:/usr/bin:/bin"; export TERM="dumb"; x2gocmdexitmessage test-50-1554468584_stDXFCE_dp24; echo "X2GODATAEND:7c6b18fd-106d-46c1-8243-477c4f699384";'"
x2go-DEBUG-../src/sshmasterconnection.cpp:1559> "channel_open_session failed.": "Socket error: Operation now in progress"
x2go-DEBUG-../src/onmainwindow.cpp:5997> Finished proxy.
x2go-DEBUG-../src/sshprocess.cpp:463> I/O error: "channel_open_session failed."" - Socket error: Operation now in progress" (4).
x2go-DEBUG-../src/onmainwindow.cpp:10759> "Searching proxy window: X2GO-test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/onmainwindow.cpp:12465> "Searching window with title: X2GO-test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/onmainwindow.cpp:10759> "Searching proxy window: X2GO-test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/onmainwindow.cpp:12465> "Searching window with title: X2GO-test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/onmainwindow.cpp:10759> "Searching proxy window: X2GO-test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/onmainwindow.cpp:12465> "Searching window with title: X2GO-test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/onmainwindow.cpp:10759> "Searching proxy window: X2GO-test-50-1554468584_stDXFCE_dp24"
x2go-DEBUG-../src/onmainwindow.cpp:12465> "Searching window with title: X2GO-test-50-1554468584_stDXFCE_dp24"
 
 
Best Regards.