Okay, I suspect this is a Plasma configuration issue then. I'm afraid you're going to have to reset your Plasma Desktop configuration, and reconfigure it again.

You can do this by moving ~/.kde4/share/config/plasma* somewhere else, outside KDE.

ok. will do it bit later and post results

thanks for giving me direction. after some more experiments i've found that problem sticks to my current activity only. after detailed investigating into plasma-desktop-appletsrc i found that plugin for this activity is groupdesktop. after changing it to "desktop" and removing all references at groups problem gone

my kde is very old (mean i didnt reinstall anything for long time, but at same time i'm on testing repositories ^) ) and i cannot remember when and why this changes occuried

thanks again for help

Not a problem. I would suggest filing a bug report against the Group Desktop layout - so the developers can investigate.