"A crash in CTR does sound like a change in what as are available, either some majors have pulled out or the algorithm thinks that something you visitors don't click on is more suitable. "
This sounds most likely.
My CTR has dropped and is similar as the OP stated his is (without divulging what it actually is).
Another big issue: 40% of traffic is mobile, so that really has to be tweaked just right. I wanted to put my ads justified to the right of the page, but that might not be showing up on some phones, so I'm going to put them back in the middle of the page.