Problems with vncserver and/or vncviewer (black screen) - SuSE linux

I want to connect to my office PC from my home PC via VNC. I ssh'd to my office PC with ssh keys and start a server: "vncserver :2 -SecurityTypes=none". Then logout and start the viewer on my home PC: "vncviewer -via 'jump host' 'server PC' :2". The viewer opens a window and tells me that I'm connected. But..... The screen is black!! Only the very first time it worked. Now nothing happens, the screen stays black. Is this a DISPLAY error? Or what do you guess. I read a lot about this problem, but nothing seems to help. --- I killed the vncserver manually and/or deleted the /tmp/.X0 or any other file of the same kind. Using "ps aux | grep vnc" shows nothing. Surprisingly, after I deleted the vnc program, I got using "echo $DISPLAY" the answer: "localhost:10.0". Why? I think the normal display number is 0. --- I want to add some new observations. I CAN start the VNC viewer if I only use the twm displaymanager in the .xstartup file on the server site. If I use startkde instead it gives me a "Call to lnusertemp failed (temporary directories full?). Check your installation." and the screen is black. I checked /var/tmp and /tmp, but there is enough space . What to do?. How do I know if Wayland is running or not?