I'm getting this issue with Roundcube 0.8-beta.
It currently only happens with IE9 as soon as I login, I get logged out with the "Your session is invalid or expired" message.
It doesn't happen to me in Firefox nor Chrome.
This is a new migrated install. I see, by googling, that many people ask "Are you behind a proxy?" when this problem surfaces.
Well yes, I am, I'm behind a Squid Caching Reverse Proxy which is also stripping SSL as well as behind a Apache director reverse proxy.
Unfortunately, in the 'googled' posts I have seen, although this question is often asked, I don't see any further reference to it? What are the issues with being behind a proxy?
I have tried the various fixes that have been suggested, but the problem still occurs with IE9. Firefox is fine (I'm using it to compose this message in Roundcube).