I used my Dolphin to copy files, and in the process plasmashell freezes. I killed it and decided to start again but keep seeing this error in a loop

QXcbClipboard::setMimeData: Cannot set X11 selection owner

What fixed it was copy another content, lets say maybe in my editor or wherever I am then i tried starting plasmashell again and it starts

This usually happens when I'm copying something from my system to KDEConnect phone directory in Dolphin

Is this in the wrong forum too??

It is either a Plasma 5 bug and you should report it on https://bugs.kde.org/ or a distribution specific bug. I don't experience such problem on KDE Neon while coping files.