I took about 3 minutes or less to type my response. There was no error message. I clicked preview and it brought me back to the page with an empty reply box. No 'You are only seeing a preview at the moment.' comment or anything. Just an empty reply box. I copied the text in again, hit preview, it appeared and hit submit.
I can confirm I have this problem too. This preview problem also happens in the wiki travel guide if you take more than a few minutes to make your edit.
Firstly, the timeouts are set to 20 minutes, so it really shouldn't be kicking in earlier than that unless something weird is going on.
Not only that, but on the forum forms it's mean to keep your session alive as long as you've got the window open.
To resolve this I need to find a way of replicating it reliably. Otherwise it's very hard to track down anything reliably. I know there have been similar reports in the past, so would definitely like it fixed.
The first incident I mentioned in this thread there was a 20 minute time lapse. But, all my reference pages were down in the dock while the forum reply form (half-written) remained in view on my desktop. The window remained open and I finished my reply.
The second incidence was just a simple reply to someone in Intros and I wrote the response without interruption in less than 5 minutes.
Especially with the health questions, I normally find the pages I need to use as references and place them in the dock - avoiding the 'hunting expedition' while in the middle of a response. But, sometimes those replies do take longer than 20 minutes to produce when I'm also adding links from the pages I have on hold in the dock. Regardless, my reply window always remains the viewable one on my desktop.
Don't know if that helps...