I'm not sure that these errors are related:
* No thumbnail available => that means your WM didn't give an xpixmap of your minimised windows (in order to have a preview of your minimised windows in the dock). Maybe a problem with the WM or X.
* about others errors: as you said, they are produced by kconquest (it's just because you launch it with Cairo-Dock and all output messages are redirected to the terminal where Cairo-Dock is launched)
* and about the warning with gedit and GMenu, it's just a warning to say that the dock doesn't like this field code ;)
I'm not sure that these errors are related:
* No thumbnail available => that means your WM didn't give an xpixmap of your minimised windows (in order to have a preview of your minimised windows in the dock). Maybe a problem with the WM or X.
* about others errors: as you said, they are produced by kconquest (it's just because you launch it with Cairo-Dock and all output messages are redirected to the terminal where Cairo-Dock is launched)
* and about the warning with gedit and GMenu, it's just a warning to say that the dock doesn't like this field code ;)
So don't worry about these messages ;)