I'm using a QGraphicView with a QGLWidget set as viewport in my multihead application, rendering of QPixmap does non't work on secondary screen.
I have destilled this problem into a small sample application, see attachment.

I'm getting these XErrors printed (same appear when running my application) once:

X Error: BadDrawable (invalid Pixmap or Window parameter) 9
Extension: 145 (Uknown extension)
Minor opcode: 4 (Unknown request)
Resource id: 0x4e0001c
X Error: GLXBadPixmap 156
Extension: 142 (Uknown extension)
Minor opcode: 16 (Unknown request)
Resource id: 0x4e00021

This may be related to the usage of the global qt_gl_share_widget() (referring to the primary application screen) which is used at various places in your GL code, but this is just a quick guess.
Note that using pixmaps w/o OpenGL works fine on secondary screens in my setup.

With kind regards, Bjoern Toschi