Could these annoying 404 errors be fixed? It's increasingly often that clicking an ad gives a 404 and reloading once (sometimes twice) loads the ad. I think the same error is causing the "An error occured" after entering the captcha, which also happens more and more often.

Are you using some load balancer and the actual web servers disagree on something? The site also feels sluggish very often compared to the past. You click and there are several seconds delay before something happens (captcha appearing, captcha solution being accepted, countdown starting, ...)