Ok, I didn’t know that 
here some things i noticed with the update: lightlyshaders effect broke to me again
and i had problem to mount dvd medias, it would show up duplicated dvd media when i tried to load one and if i tried to click it from file manager to load it would unmount after that, i had to log on a xfce session to burn a backup dvd
Plasma 5.27.6 is in the Arch repos - https://kde.org/announcements/plasma/5/5.27.6/
Fixes include “Panel: mitigate plasmashell high CPU usage when moving windows.”
Got it now … 
thanks for pointing to. just updated 
I have been running Wayland for a while also on KDE Plasma and no issues.
i did not tried Wayland. Last time i want to login into Wayland-Session Plasma didn’t start. Was about 5 Months ago or so.
I should try again sometime
That’s weird If it happens when you try it again, it might be worth filing a KDE bug report. They’re planning to make Wayland the default for Plasma 6.
I have tried again. Still the same. I can login with my actual user account but just a black screen. I have tried it with lts and arch kernel. i will have to create a new user account
Yes, try that.
i am a 100% sure that will work. last time i did the same.
then I have to set up everything completely new again…
nfs-server new. firewall rules i can take with me, that should be no problem. and just reinstall every single program. that’s a big task
You should be able to copy over some of the files from .config and .local/share to make that task easier.
Why would you need to install every single program? Surely most/all are installed per system rather than per user?
That’s right. It must be possible. I will sit down the next few days times on it
You are right. My fault. Where was my head 
Yes, it’s not something to rush 
Is there any way to find out why plasma won’t start with my current configuration under Wayland?
Maybe just an extension which is buggy under Wayland or a theme. I am currently using Nordic Bluish
I’m not an expert on logs, so wouldn’t know whether the answer is hiding in one of them.
I’m not so sure it would be a theme, but it could be an extension. My slow labourious process of doing it would to be to create a new user account, and slowly reconfigure it until it happened again (or not).
It might be worth asking the question at KDE Discuss to see if anyone there has an answer/explanation.
Ok, that makes sense.
i will try my luck with this.
Anyway, many thanks for your support. I will find a solution. As soon as i know more i will let you know
Yes, please do that.