I have the exact same problem!
In my INBOX (with >26000 messages) nothing happens if I try to open (double click) any of the messages, I get bumped back to the login screen after a while, just as you described. (Marking a message and clicking Reply or Forward works fine!)
Other than that the performance is much better now. If I could only open my mail too I would be able to switch to RC. :-)
Regards, Håkan Lindqvist
(Sorry about not being able to reply properly for those who use threading MUAs, but I wasn't subscribed before and it didn't appear to be possible to get the mail headers out of the archive.)
Another thing, before i forget. At this point, it seems that roundcube has issues opening individual mails in large mailboxes. For exmaple, my sent mailbox has about 3500 messages, can't open any messages from there (ranging from 1k to 11MBs), nor from one of the mailing lists that contain about 16000 messages. it just sits there saying that it's loading. eventually it times out and kicks me back to the login screen stating that the session has expired.
can anyone confirm this?
cheers, rodolfo