What is this diagonal grid on the faces of my active group?

Thanks Dave.
SU Pro 2017, i updated my profile.
No, it’s not hidden geo.
The file is big, i think i have to upload it somewhere else.
What i did realize: It is not on all geo, but only on one(some?) group(s). Have to check deeper.
Also, it does not show on all zoom levels.
the stuff with the grid is also not showing correct in LO: