Recently, I became aware of this troublesome issue in regards to document previews with FAST Search for SharePoint. It was on a new installation of SharePoint. I had everything indexing great and my search results looked good. However, after a while, I noticed that document previews were not appearing for our documents stored in SharePoint. After further investigation and some discussion with some colleagues, we discovered that FS4SP had an issue with Claims Based Authentication as described in KB2641517. The article describes the issue and makes it pretty clear what you have to do to make it work: switch back to classic authentication. I wanted to go into a bit more detail about what is involved in that work-around though.
If you are familiar with claims at all, you may know that you can switch a web application from classic authentication to claims, but you may not switch back. That means any work-around you pursue can be a bit involved. Before, you start ripping your farm apart though, it is best that you understand where the issue with claims is. In reality, it is not an issue with the web application that hosts your search center. It can actually be running claims and document previews will work. It’s the web application that hosts the documents themselves. For example, if you have web application A running claims and web application B running classic, you will get document previews for the documents on web application B even if the search center was running on web application A. The reason I point this out is so that you don’t just try throwing your search center on a classic authentication web application hoping that it will work.
That means if you want to get document preview working, you have to detach your content databases, delete the web applications, reattach the content databases, redeploy your solution packages and hope for the best. :) It really shouldn’t be that big of a deal, but you should definitely plan ahead if you are going to make that kind of change. From my understanding it has something to do with how the claims based application talks to Office Web Apps. Office Web Apps works fine with claims based web applications when it comes to viewing / editing, so I don’t know why it’s an issue here. I’ll let an expert on claims answer that. Anyhow, I hope this insight helps should you run into this issue.