Todays updates didn’t help.
When the black window is served, I kill the server via ssh and restart it. Then the window is served again. But starting Firefox within seconds stops with no error message, but restarting does not work, error message given in post above.
Starting the vncviewer in a console when serving the black window only gives:
vncviewer 192.168.188.1:9
TigerVNC Viewer 64-bit v1.8.0
Built on: ??-??-?? ??:??
Copyright (C) 1999-2017 TigerVNC Team and many others (see README.txt)
See http://www.tigervnc.org for information on TigerVNC.
Sat Jul 8 19:15:07 2017
DecodeManager: Detected 2 CPU core(s)
DecodeManager: Creating 2 decoder thread(s)
CConn: connected to host 192.168.188.1 port 5909
CConnection: Server supports RFB protocol version 3.8
CConnection: Using RFB protocol version 3.8
CConnection: Choosing security type VeNCrypt(19)
CVeNCrypt: Choosing security type TLSVnc (258)
Sat Jul 8 19:15:15 2017
CConn: Using pixel format depth 24 (32bpp) little-endian rgb888
CConn: Using Tight encoding
CConn: Enabling continuous updates