Tried both AF lenses I have, standing in front of a "pretty" 70s wallpaper in dim-ish light. Left most sensor hunted, and hunted, and hunted. Right most hunted, but found focus. All sensors apart from the center one focused pretty fast on the wallpaper. The center one did not. At all. It'd hunt, and hunt, and hunt, or even get stuck at one of the ends, trying to move further in that direction every time I press AF.
I didn't want to touch the sensor window, but I blew into the camera with mirror up (and dust flew my way!), and I cleaned the mirror, which was quite dusty as well. Now it focuses again! Yay!
Thank you very much