Yes, the issue still persists and I’m trying to find a solution for that.
Thanks to the ydar’s hint creating a new user account and then successfully confirm the thumbnail creation process, it is clear, that the issue is not a fundamental system wide problem, rather than a user configuration one.
It is also clear, that the whole process of creating thumbnails has stopped on my machine because of not seeing any wallpaper thumbnails at Wallpaper in Apperance & Style in System setting
Ouch - then when you’re not busy, try another one with the whole ~/.config to ~/.configFUKIT folder …
Then waste a while moving it all back to the newly created .config folder (dualpane dolphin) piecework… trying to select exactly half of everything you see each time, ending with the hidden junk.
I’m very, very happy to see such much engagement to all of you to solve the issue 
Never underestimate the fascination of Plasma. We will waste entire weeks trying to nail down endlessly diminishing returns…
I was serious about the ICONS change, however… do that - there are two times when I fixed ‘niggly’ issues in Plasma with apparently completely unrelated and meaningless acts.
Never underestimate Ghosts in the Machine.
How nice for you;
and if you bothered to respond to my post, I might be joyous beyond measure. 
I believe that’s… Phuket. 
Hello soundofthunder,
Ideally, just creating the folders should have been enough, rather than assigning permissions.
I simply just created the folder /thumbnails/ and compared it’s permission to other folders in ~/.cache/ to be shure it’s set correct by default - I’ve not done any permission changes.
I’ve noticed that also the creation of wallpaper thumbnails has stopped.
That’s what I get at System Settings
Folders
~/.cache/thumbnails/large and
~/.cache/thumbnails/normal
are empty.
have you installed any
dolphinplugins/extensions?
No
see what happens if you completely remove the
~/.cache/thumbnails/directory
No effect
and if you bothered to respond to my post, I might be joyous beyond measure.
I’m sorry about the delay and hoping your joyous now goes beyond measures.
Ok, the time and energy it took in trying to solve the issue - many thanks to all for your kind support - and the time and energy it will take is far beyond the time to reinstall Manjaro.
So, the issue’s state is unsolved but closed for me and I’m happy to see Manjaro’s forum responses very prompt and friendly - even at night
It looks like you tried really hard to respond to every line, except for my instructions which you seem to have missed completely…
So, here they are again;
I suggest the following procedure:
1. Logout
2. Enter a TTY from the login screen
3. rm -rf ~/.cache/* (sudo not recommended)
4. Reboot.
The ~/.cache directory is safe to delete and will be progressively rebuilt as you use your computer.
See how things stand after doing that.
Regards.
Abandoning the topic like this is unfair on those who have tried to help. This topic will indeed be closed.
We wish good will and 
on you.
Please excusa my accent.
Sadly not reciprocated; perhaps worth noting that this user simply wishes to persuade people to engage, but does not wish to offer feedback on responses… essential for a technical forum.
I’ll add this here just so the OP knows that it is an issue that only occurs on Plasma X11, and is not Manjaro-specific. The bug reporter is using Arch Linux, but there is no other feedback (as yet) on whether the issue is restricted to Plasma X11 on Arch-based distros, or is a general Plasma X11 issue:
