I recently started experiencing this and came to the forum to post the same issue. netmeg, I’m wondering if unchecking/disabling “Enable click and impression tracking for this advert” on an ad makes it appear again. That is what worked for me.
Arnan, I have looked through your code and did some debugging. I’ve at least narrowed it down to the adrotate_fallback() function. For some reason, all of my ads, except the ones with tracking disabled, are going through that function and catching on the if statement for ‘expired’. (referring to adrotate-output.php on line 590)