My b2evolution Version: Not Entered

I'm running b2evolution 1.9.3 on a standard LAMP system, and I've run into a very weird problem (which doesn't occur on my other b2e system running the same software, on the same ISP). If, while writing or editing a post, any of my users attempts to preview their post, they're logged out the next time they try to do anything in the b2e interface.

I did a bit of research with tcpdump and Wireshark, and I see that when the preview page is displayed (in a new window, in case that's relevant), a new cookie is set, so it's not particularly surprising that things are messed up whenever they try their next request. What I'd like to know is, why is this cookie being set, and how can I work around this issue?

Thanks,
Alex Kirk