I'm not sure what happened, I was high up in the air in the 3D editor to get a full overview of my map, and then wanted to go down again. Since due to all the detail the FPS was very low, I figured I should reduce clip first so the FPS would drastically increase and movement would increase as well. I pushed the -Clip button many times while going down, and then everything became a very weird blue colour. I closed the 3D editor window, reopened it, and it put me at the starting position, but with maximum clip it seems. So FPS is now like 0.5, and the -clip button doesn't seem to do anything anymore!
Could it be possible that it actually went beyond zero clip when I pushed the -clip button a lot, so that it started at the highest possible value of the numerical Clip variable? Like, it's somewhere in the millions now, and hence clicking on the -clip button doesn't seem to do anything anymore? I can't work like this, with 0.5 FPS. Is there any way I can reset the clip amount to a normal level, like manually in a settings file somewhere?
EDIT: A reinstall solved the issue. Sorry to bother you!