I'm not experiencing what you're experiencing. The only comments I can think to make that might be remotely relevant, that might trigger something relevant in someone else's brain:

1. Cookies (I think) are a royal pain on the forums for me. (In general, on the computer, using Chrome on Mac OS X.) I can sometimes barely compose a response before I'm "logged out" (meaning that when I try to post a response, if it's been more than just a few minutes, I'll get the "you're not logged in" message. I use LastPass in Chrome and have this password set to auto-log in, and since there's a log in field on every page of the forum, my typical MO is to hit any page on the forum, let LastPass auto log me in, and then I can go where I wanted to go. When posting a response, fortunately the forum typically holds my post in memory so that when I hit the "you're not logged in" page, LastPass kicks in and auto-logs me in, then the forum is set to redirect to actually posting my message. I have lost a few messages, but usually they get through in this way.

2. I know I have a different password for the entourage main site than I do for the forums. My failing memory is vaguely recalling something about us needing to change passwords to the main site (our entourage account, not our forum accounts) because Entourage changed something on their end. So, I believe the two systems are different on some level, since they use different login accounts (which maybe a lot of ppl just have set to be the same thing), and may very likely work in different ways.

Those are the only two things I can possibly contribute, and maybe those points are related in some way to your issue.