So that's why Krita was never fixed, it only happens on xfce apperently because it handles the clipboard differently. Since Krita is developed as part of KDE they probably don't care enough to ever bo...
Also the ENV he mentions "XFSETTINGSD_NO_CLIPBOARD" doesn't work... XFCE seems to ignore it and starts the clipboard manager anyways. However, if you see it in your dash and right click and select quit yourself before opening Krita it wont crash on copy paste anymore.