I also kept getting "session expired" errors after upgrading to 0.1-stable. I could login, view an email or two, and would then be disconnected after a few minutes.
In my case, the problem was caused by bug #1484819
http://trac.roundcube.net/ticket/1484819. After applying the regression described there by sirocco, everything is working again.
Looking at the log of my IMAP server (CommuniGate Pro), I can see that Roundcube would attempt to login via IMAP using CRAM-MD5 authentication, fail, fall back to LOGIN authentication, succeed, and perform one operation. This would repeat for several operations, until finally the IMAP server says "too many failed login attempts" and blacklists the IP (localhost) for a short time, so the LOGIN authentication fails as well. Roundcube would then, understandably but incorrectly, report "session expired".
This may be one of several reasons why so many people are seeing the "session expired" problem with the new release.
Franzzup