Nvidia: weird behavior between 2 sessions

I recently (at last) installed nvidia driver without problem (CL: 35930, thinkpad P1, Xeon 12, quadro P2000).
Many thanks for the excellent tuto.
I have 2 sessions. When I move from one to the other, the desktop has a weird behavior: icons and voyels are missing (see pictures).
Does somebody has encountered this situation or has any idea to solve this?