It seems that the nouveau driver is not fit for modern KDEs use of OpenGL [1] as suggested in a comment to a different bug [2].
In my case the affected card is a GeForce GT 710 (MSI 1 GB D3 X GT710 LP passive). [1] https://bugs.freedesktop.org/show_bug.cgi?id=98039#c3 [2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=851416#10
