Comment # 7 on bug 1091245 from Michal Srb

(In reply to Sergio Lindo Mansilla from comment #6)
> Created attachment 770269 [details]
> tcpdumpt.out

Thank you. The tcpdump shows that XDMCP negotiation went fine and then some X
clients (presumably GDM greeter and other helper processes) connected to X
display number :1 (that is Xvnc). However none of the X clients did any
rendering, just queried few extensions and atoms, which looks like GTK
initialization.

I would suspect that something is stuck waiting in GDM, but you said that you
observed the same issue also with lightdm. However, that was after upgrade, so
maybe something else was broken in that situation and we are seeing two
different problems.

Can you please try with just another display manager, perhaps XDM or KDM?

> Because there is still a bug, fixed on upstream Yast and not yet in
> Tumbleweed 20180511, that those packages are not installed and then yast's
> "Remote Administration (VNC)" complaints that no services were found for
> tigervnc and tigervnc-http to configure for firewalld.

I saw that error too, but it should not matter after disabling the firewall.