I am using KDE Platform Version 4.14.8 on top of VNCViewer on windows.

It happened to me a few times recently that all of my windows disappeared suddenly. top and ps shows all the related process are gone. The only relevant thing that I can google is:
viewtopic.php?f=66&t=117639

I didn't press Windows + S, I have disabled 'Windows + S' as a shortcut for 'Stop Current Activity' already in 'Global keyboard shortcuts'.

Somehow my mouse is big sluggish, I am wondering if it triggered some kind of key combination for short cuts like 'Windows + S'.

What other things can I do to prevent windows disappearing? This is very destructive to my work.

Thanks.

Looks like there is a core dump of KDE at time when windows disappear:
core.124704: ELF 64-bit LSB core file x86-64, version 1 (SYSV), SVR4-style, from '/usr/libexec/gnome-terminal-server', real uid: 16778781, effective uid: 16778781, real gid: 16777729, effective gid: 16777729, execfn: '/usr/libexec/gnome-terminal-server', platform: 'x86_64'

You seem to have a very confusing and rather outdated installation: Why do you use such an outdated Plasma version? And why do you think a core dump of a Gnome application, which has no relation to KDE , is the culprit?

FWIW: Plasma 4.x is not supported anymore