I've had this problem often lately, I dont know what button I pressed accidentally, or what could cause it.
Anyway, all objects in the scene start to flicker if I do that particular unknown action,
I can see their wireframe and their polys but they are flickering and half-invisible
if I set the viewport to camera the flickering disappears, but if i set it to user, it flickers again.
Does anybody know how to solve the problem or did I just activate a strange camera-mode?

Umm isn't that because you've got 2 objects with a 2 polys at exactly the same place to eachother? You probably know that. Anyways it's cus it's kinda 'fighting' for space, so when you move the camera it causes flickering.

I've had this problem in Mod Tools 4.2, if you're in Mod Tools 6.0, then it might still exist. Try, Freezing the Operator Stack, Closing XSI, then reopening it. It could be eating up too much Memory. If that doesn't work, triangulate it, then export it as .obj or .emdl and re import it. That usually fixes the rendering problems.

Oddly enough, I dont have this problem in XSI 5.0, even though its on the same laptop running the same Integrated graphics. I always assumed it was because of the Graphics Chipset, but since I have yet to experience it in XSI 5.0 it must be a Mod Tools related issue.

Hover your mouse cursor over the problem viewport and press R to reset the camera and see if it helps.

I also use to get this problem, but i just push "a" and then the flickering disappears.