My title is just what I was thinking at the time. No this is not about adsense. I, along with many are struggling with spiraling SERP changes. I have about 12 quality sites. Not junk - all unique and quality content. Albeit all are in very competitive markets. UP to about 3 weeks ago to the day I could count on the daily traffic producing a certain amount of adsense revenue - thus my measuring stick for what was happening that day. First, one of my best sites lost 90% of its traffic. The next week #2 got hit with traffic loss. Another one of my sites climbed a bit but not enough to makeup the revenue lost on the others. Yesterday all sites were in the toilet. Revenus is 25% of where it was 3 weeks ago. Is it possible since all sites reside on one server, there is something else going on here? The fact is, I stand by these sites as top quality and SEO done well. Worried, confused, mad.... - not always in that order.
If the sites are all high quality, I don't think there is any problem having them on the same server. If you're on shared hosting, it's possible one of the other sites on the same server isn't quite so legitimate and is hurting your standing with Google. That's unlikely though. It's more likely just a change in the algorithm that's hit you hard. Have you tracked the keywords your ranking for to see if you've just dropped down the rankings or disappeared altogether?