I'm having the EXACTLY sam issue. I'm having almost 1000 users and now Help Desk is crowded with their yells and shouts. :-( Please, did you find the solution? DotnetNuke team? Anyone?
web.config has authentication set to FORMS:
<forms name=".DOTNETNUKE" protection="All" timeout="60" cookieless="UseCookies"/>
Authentication module is enabled:
<add name="Authentication" type="DotNetNuke.HttpModules.AuthenticationModule, DotNetNuke.HttpModules.Authentication" />
The mysterious part is that when I create new user in AD, he CAN log to DNN site. BUT, after a custom period (ranging from couple of minutes to the end of session), the user get logged out. And when he opens a new browser and browses to DNN site, he always has to enter his credentials.
And the last, I have been logged on to DNN with my user account for a while month now. I never get logged out. And it's just me out of all users.