When enabling VNC with current factory, X fails to start with this backtrace:

[    30.576] (EE) 
[    30.576] (EE) Backtrace:
[    30.576] (EE) 0: /usr/bin/Xorg (xorg_backtrace+0x79) [0x562f82784949]
[    30.577] (EE) 1: /usr/bin/Xorg (0x562f825d3000+0x1b5619) [0x562f82788619]
[    30.577] (EE) 2: /lib64/libpthread.so.0 (0x7efe94f98000+0x11f70)
[0x7efe94fa9f70]
[    30.577] (EE) 3: /usr/lib64/xorg/modules/extensions/libvnc.so
(vncRandRGetOutputCount+0x47) [0x7efe92e8add7]
[    30.577] (EE) 4: /usr/lib64/xorg/modules/extensions/libvnc.so
(_ZN14XserverDesktop19computeScreenLayoutEv+0xb1) [0x7efe92e8d471]
[    30.577] (EE) 5: /usr/lib64/xorg/modules/extensions/libvnc.so
(_ZN14XserverDesktop14setFramebufferEiiPvi+0x93) [0x7efe92e8d913]
[    30.577] (EE) 6: /usr/lib64/xorg/modules/extensions/libvnc.so
(_ZN14XserverDesktopC2EiNSt7__cxx114listIPN7network11TcpListenerESaIS4_EEES6_PKcRKN3rfb11PixelFormatEiiPvi+0x20f)
[0x7efe92e8dc2f]
[    30.577] (EE) 7: /usr/lib64/xorg/modules/extensions/libvnc.so
(vncExtensionInit+0x364) [0x7efe92e83bc4]
[    30.577] (EE) 8: /usr/bin/Xorg (InitExtensions+0x89) [0x562f8269be09]
[    30.577] (EE) 9: /usr/bin/Xorg (0x562f825d3000+0x5efeb) [0x562f82631feb]
[    30.577] (EE) 10: /lib64/libc.so.6 (__libc_start_main+0xeb)
[0x7efe94bfafeb]
[    30.577] (EE) 11: /usr/bin/Xorg (_start+0x2a) [0x562f8261bf6a]
[    30.577] (EE) 
[    30.577] (EE) Segmentation fault at address 0x0
[    30.577] (EE) 
Fatal server error:
[    30.577] (EE) Caught signal 11 (Segmentation fault). Server aborting

Reproduce with:

enable libvnc with default parameters in /etc/X11/xorg.conf.d/10-libvnc.conf
switch to kdm (sddm doesn't support XDMCP)