All of registered users, host user too, can't login to the web site through internet , however, they can login normally on the server itself. 

The problem started with limiting session time to three or less minutes for some users, last the session for all users are killed immediately.

Notice: the event viewer shows that internet login occurs successfully!!! but real authentication can not be acted.

It seems that DNN doesn't store user session or something related to the firewall or network.

 

Thanks Sebastian,

 

I didn't limit the session, it reduced on itself.

 

Now no one, also host admin, can login to the website except on localhost.

 

There a termination of login session because the event viewer shows that login through internet passes successfully.

Are the users connecting to the internet via a wireless access point? We ran into a situation in our office where a software update for the Wireless guest accounts set the session timeouts very low.