So, I'm like others, can get preview pane to work with a new user to RCmail, but not an existing one.
I tried everything noted in this thread, deleting all the rows from the cache table with no luck. I then went into the users table to see that the preferences of my old user had much more information in it than the new user. I saw that one of those elements was preview_pane (it looked like this "preview_pane";b:0;s:14), well, not caring about other things in that field, I did a sql update on the preferences field for my user that replaced my old preference with what's in the new user's preference. *voila* preview pane!
I'll bet that you could just copy the entire preferences field of the old user and change the "preview_pane";b:0;s:14 to "preview_pane";b:1;s:14 and it would allow it, that's just a guess, but I'm betting that b is a boolean...