Comment # 6 on bug 1091245 from Sergio Lindo Mansilla

Created attachment 770269 [details]
tcpdumpt.out

Hallo Michal,

Perhaps something related, since you didn't mention it.
Did you have to manually install the package "xorg-x11-Xvnc"?
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 also get black screen with the firewall disabled.

Please, find output file attached.