I will do that.

Thank you for your great help!

I ve got the same problem as topic starter. But I can't find file .xprofile on my computer what to do?

It works now. Added russian locale in System Settings->Country/Region & language. For two years it worked without. Maybe update or smth.

This happened on my system before, and I forgot what I did to fix it.
Anyway, now it's back, just after the upgrade to 4.6. I have been using ibus (for Chinese input) all the time, but it has definitely to do with it: When I just now restarted ibus (right click on the icon, Restart), immediately copying something into the clipboard from kwrite with CTRL-C worked again.

My best guess is, that it has to do with the startup order. Probably before the 4.6 upgrade, ibus was started *after* some KDE component that handles keyboard shortcuts, and now it gets started *before* (for whatever reason). I don't know how to investigate this further, though - but it's surely annoying, and I hope someone with more knowledge can do that.

Thanks,
Patrick.