Package: x2goserver
Version: 4.0.1.19-0x2go2+git20160126.1064+jessie.main.1

Package: x2goclient 
Version: 4.0.5.1-0~1167~ubuntu14.04.1

After I rebooted my server, I started getting:

Connection failed. xrdb: Can't open display ''

from the client. If I run "x2goclient --debug", here's the output from the Debug:

x2go-DEBUG-../src/onmainwindow.cpp:2682> Creating QPixmap with session icon: '":/img/icons/128x128/x2gosession.png"'.
x2go-DEBUG-../src/onmainwindow.cpp:1286> Removing apps from tray
x2go-INFO-8> "Starting connection to server: myserver.org:22"
x2go-DEBUG-../src/onmainwindow.cpp:2783> Starting new ssh connection to server:"myserver.org":"22" krbLogin: false
x2go-DEBUG-../src/sshmasterconnection.cpp:173> SshMasterConnection, host "myserver.org"port 22user "myuser"useproxy falseproxyserver ""proxyport 22
x2go-DEBUG-../src/sshmasterconnection.cpp:210> Starting SSH connection without Kerberos authentication.
x2go-DEBUG-../src/sshmasterconnection.cpp:214> SshMasterConnection, instance SshMasterConnection(0x27c1250)  created. 
x2go-DEBUG-../src/sshmasterconnection.cpp:450> SshMasterConnection, instance SshMasterConnection(0x27c1250)  entering thread. 
x2go-DEBUG-../src/sshmasterconnection.cpp:486> libssh not initialized yet. Initializing.
x2go-DEBUG-../src/sshmasterconnection.cpp:797> cserverAuth
x2go-DEBUG-../src/sshmasterconnection.cpp:812> state: 1

x2go-DEBUG-../src/sshmasterconnection.cpp:983> Challenge authentication failed. Trying password mechanism if available.

x2go-DEBUG-../src/sshmasterconnection.cpp:989> Password mechanism available. Continuing.

x2go-DEBUG-../src/sshmasterconnection.cpp:993> Password authentication requested.

x2go-DEBUG-../src/sshmasterconnection.cpp:662> User authentication OK.
x2go-DEBUG-../src/onmainwindow.cpp:2878> SSH connection established.
x2go-DEBUG-../src/onmainwindow.cpp:3160> Continue normal X2Go session
x2go-DEBUG-../src/sshprocess.cpp:199> Executing remote command via SshProcess object 0: "export HOSTNAME && x2golistsessions"
x2go-DEBUG-../src/sshprocess.cpp:204> this=SshProcess(0x27efcc0)  Running masterCon->addChannelConnection(this, ' "3c34e9d5-a896-4976-a52a-12cd3dfbb0fd" ', ' "bash -l -c 'echo "X2GODATABEGIN:3c34e9d5-a896-4976-a52a-12cd3dfbb0fd"; export PATH="/usr/local/bin:/usr/bin:/bin"; export TERM="dumb"; export HOSTNAME && x2golistsessions; echo "X2GODATAEND:3c34e9d5-a" '); 
x2go-DEBUG-../src/sshmasterconnection.cpp:1312> Locking SSH channel connection MUTEX.
x2go-DEBUG-../src/sshmasterconnection.cpp:1314> Passing new channel conenction object to channelConnections.
x2go-DEBUG-../src/sshmasterconnection.cpp:1316> Unlocking SSH channel connection MUTEX.
x2go-DEBUG-../src/sshmasterconnection.cpp:1504> Creating new channel.

x2go-DEBUG-../src/sshmasterconnection.cpp:1508> New channel:0x7fbcd8011f80

x2go-DEBUG-../src/sshmasterconnection.cpp:1539> Executing remote: "bash -l -c 'echo "X2GODATABEGIN:3c34e9d5-a896-4976-a52a-12cd3dfbb0fd"; export PATH="/usr/local/bin:/usr/bin:/bin"; export TERM="dumb"; export HOSTNAME && x2golistsessions; echo "X2GODATAEND:3c34e9d5-a896-4976-a52a-12cd3dfbb0fd";'"

x2go-DEBUG-../src/sshmasterconnection.cpp:1562> New exec channel created.

x2go-DEBUG-../src/sshmasterconnection.cpp:1666> EOF on channel 0x7fbcd8011f80; SshProcess object: 0
x2go-DEBUG-../src/sshmasterconnection.cpp:1734> EOF sent.
x2go-DEBUG-../src/sshmasterconnection.cpp:1738> Channel closed.
x2go-DEBUG-../src/sshprocess.cpp:517> SSH finished: raw output (stdout): "
X2GODATABEGIN:3c34e9d5-a896-4976-a52a-12cd3dfbb0fd
X2GODATAEND:3c34e9d5-a896-4976-a52a-12cd3dfbb0fd
"
x2go-DEBUG-../src/sshprocess.cpp:523> Have stderr only, something must be wrong.
x2go-DEBUG-../src/sshprocess.cpp:528> SSH finished: false - "xrdb: Can't open display ''
" (0).
x2go-DEBUG-../src/onmainwindow.cpp:3549> "xrdb: Can't open display ''
"
------------------------------------------------------------------------------------------------------

I tried with a different server and it seemed to work fine:
Package: x2goserver
Version: 4.0.1.19-0x2go1

James Klaas