Veyon 4.2.2 severely unstable on Kubuntu 16.04

[robertc]

We just tested veyon 4.2.2 for Ubuntu 16.04 on our Kubuntu 16.04 configuration, which has all updates currently available in its repositories. The test room has 1 computer for the teacher and 14 for student's.
Unfortunetely, the software is so unstable with this configuration that it seems impossible to use it in a real class room.
We are using the Builtin VNC server (x11vnc).
The functionality we most seek is displaying the teacher's screen on the student's screens. We found no 100% reproducible way to crash the software, but playing with the Monitoring/Fullscreen demo/Window demo/Remote View/Remote Control functions during 1 or 2 minutes (sometimes selecting one computer, sometimes all) is usually enough to get a crash.

By starting the software from a terminal, we were able to see several kinds of error messages appearing very often, although not systematically leading to an immediate crash:

QIODevice::write (QFile, "/tmp/VeyonMaster.log"): device not open

2019-05-28T13:33:44.575: [WARN] QTimeLine::setDuration: cannot set duration <= 0

2019-05-28T13:38:02.082: [ERR] VeyonConnection: unknown message type 6 from server. Closing connection. Will re-open it later.

2019-05-28T13:38:18.424: [ERR] Last message repeated 4 times

2019-05-28T14:01:23.853: [WARN] VariantStream: none or invalid data read

2019-05-28T14:01:23.854: [ERR] VariantArrayMessage: invalid message size 538444031

Here is a full sequence leading to a crash:

QIODevice::write (QFile, "/tmp/VeyonMaster.log"): device not open
2019-05-28T13:47:36.999: [WARN] QTimeLine::setDuration: cannot set duration <= 0
QIODevice::write (QFile, "/tmp/VeyonMaster.log"): device not open
2019-05-28T13:47:59.997: [ERR] VeyonConnection: unknown message type 98 from server. Closing connection. Will re-open it later.
(crash here)

A few weeks ago, we did some tests with Kubuntu 18.04 and an earlier 4.2.x Veyon version, but did not encounter those problems. Is there any way to make Veyon 4.2.x really usable also on Kubuntu 16.04, as an upgrade to Kubuntu 18.04 is currently not possible for us?