It looks like this might indeed be the result of a KDE bug, as mentioned by @Aragorn in the previous thread.
Good news: The KDE bug seems to have been fixed.
- 500038 Attempting to switch user via Kicker caused Plasma Shell to crash
Bad news: It’s fixed in Plasma 6.4.0 – you might have to wait a little bit longer before you can reap the benefit.
In the meantime, I suggest using the workarounds mentioned in the thread you referenced; it’s only a temporary inconvenience.
Regards.
The commit from KDE for anyone interested;