Thank you so much Patrick, yes I'm using testing. 
I downgraded  libkdecorations2-5v5 and now it works fine.
Kind regards. 
On Mon, 18 Oct 2021 22:45:17 +0200 Patrick Franz <
deltaone@debian.org> wrote:
> Hi,
> 
> I assume you're using testing. Unfortunately, Plasma is broken in 
> testing at the moment if you have upgraded libkdecorations2-5v5 to 5.23. 
> Could you check that ?
> 
> If this is the case, I wrote about a possible solution here:
> 
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996726#10> 
> 
> -- 
> Med vänliga hälsningar
> 
> Patrick Franz
> 
> 
> 
>