[winswitch] rfb mode not working

[Claudio Negri]

Hello,
I have installed xpra v4.0.2-r26625 in Debian GNU/Linux 10 (buster).
I'm starting it with this command:

xpra start-desktop --daemon=no --tcp-auth=multifile:filename=/root/secret
--bind-tcp=0.0.0.0:3013 --html=on --pidfile=/run/xpra.pid
--start-child=xfce4-session --exit-with-children=yes --bind-rfb=0.0.0.0:5901

Connection with xpra attach is ok. Connection with HTML5 client is ok.
When I try to connect with vnc client on port 5901, it loads the "first"
screen, but the screen never updates. Actions (mouse clicks, drags, ...)
works correctly, but I have no feedback: the screen is freezed.
Disconnecting and reconnecting "refreshes" the screen, but usually at the
third connection attempt, the xpra server dies with Segmentation fault:

2020-06-16 13:53:32,090 xpra is ready.
2020-06-16 13:53:32,091 xpra GTK3 X11 desktop version 4.0.2-r26625 64-bit
2020-06-16 13:53:32,213  uid=0 (root), gid=0 (root)
2020-06-16 13:53:32,214  running with pid 12173 on Linux Debian 10 buster
2020-06-16 13:53:32,214  connected to X11 display :0 with 24 bit colors
2020-06-16 13:53:32,215  initial resolution: 1856x704
2020-06-16 13:53:32,218   :0.0 (491x186 mm - DPI: 96x96) workarea: 1856x673
at 0x31
2020-06-16 13:53:32,218     monitor 2
2020-06-16 13:53:32,917 OpenGL is supported on display ':0'
2020-06-16 13:53:32,918  using 'llvmpipe (LLVM 7.0, 256 bits)' renderer
2020-06-16 13:53:52,717 RFB version 3.8 connection from 10.0.0.1:40238
2020-06-16 13:53:52,910 Warning: client did not supply any modifier
definitions
2020-06-16 13:55:16,220 xpra client 0 disconnected.
2020-06-16 13:55:21,778 RFB version 3.8 connection from 10.0.0.1:40250
2020-06-16 13:55:21,906 Warning: client did not supply any modifier
definitions
2020-06-16 13:55:54,983 xpra client 0 disconnected.
2020-06-16 13:55:59,939 RFB version 3.8 connection from 10.0.0.1:40258
Segmentation fault

Thanks,
Claudio Negri