After using Konsole in KDE 5 for some time, the copy text function stops working.
Ctrl+Shift+C acts as Ctrl+C (prints ^C in Konsole) and menu item Edit->Copy is greyed/grayed.
Defined alternative hot key Alt+Shift+C, but that does not work either.

Paste, both menu item and Ctrl+Shift+V, works without any problem.

Restarting Konsole only won't fix the problem.
An old (earlier started) Konsole still works with text copy.
Since the problem pops up, any newly started Konsole has the same problem and symptoms.
Closing all Konsoles and starting fresh new one won't help too.

Any idea what the heck could fix this, apart of logout/re-login to X?

Update: reboot of the system __does__ fix this problem.

(Kubuntu 16.04, distro default KDE Plasma 5.5.5)

Bottom line: It is really somewhat annoying, in a situation when one has 10+ remote ssh sessions open doing a complex reconfiguration of a distributed system...

I have no idea what might be causing that, but what you might try is starting Konsole with a debug or verbose option (assuming it has one) from another instance and see if that will give you any more info about what is going on

Thanks for that hint buddy - unfortunately no verbose or debug command line options.

My suspect is some sort of memory corruption inside tha app - overwrites status/config variables.

There is --list-profile-properties option, but this one does not list the actual value of parameters, just variable type.
Another one gives possiblility to set them, but no way to get the current value. Pretty much stuff controlled via menus anyway, so one can see and set the parameters there. Well, at least some of them.

Also did not find any konsole specific config file in ~/.kde, even no occurrence of string "konsole" or "Konsole" in any file in that folder.

You could try installing gdb and then run konsole through that.

:-) I wish I had time to debug konsole! and participate more in free projects!

Started using terminator eventually. It's a kinda paradox - non-native terminal emulator integrates better with KDE than konsole... layouts survive hard crash of the X/KDE/Plasma and all windows incl tabs nicely pop up after re-login. And No copy / paste issues.

That's what I use!

:-)