I recently got forwarded this message from a customer that had migrated to SSL.
Google has detected structured markup on some of your pages that we believe violates our structured data guidelines. Because incorrect information lowers the quality of search results for users, rich snippets will not appear in Google Search results for pages with incorrect structured data markup. This manual spam action has been applied to domain.com. We suggest you update the incorrect structured data and file a reconsideration request once you have fixed your site. After we determine that you have complied with our guidelines, we will remove this manual action.
What is really interesting here, is that this is a lot of text, but gives absolutely no clue as to what the issue is. One day, all is fine, the next day, this message.
No errors were found in the rich snippets, no spam was added whatsover. On some forums I found some suggestions that maybe some data was hidden, or there was possibly no link to submit reviews, or to a listing of all reviews. This was all not the case here.
What I noticed here is that the Google Webmaster tools registration had only one property registered: the http://www variant. And Google webmaster tools wants you to register all your domain variations. So we quickly added the https variants, and set one of them as primary.
This seems to have solved the issue.