I have a web forms application that I tested and works good locally. When i upload it to a web server that hosts my site, it often just logs out users.
More than a month now, this problem is created - without reason users get out of the system and must log in again.
Sometimes get out again after a minute - sometimes after a few minutes
The loggin issues are connected with the cookies that you set. Usually a move from www. to non www. can make the cookie lost if domain is not set,
or a move from secure https to non secure http and vice versa.
Make sure that you have set the domain (with out the www.) on this sections on web.config as:
<httpCookies domain="domain.com" />
<roleManager domain="domain.com" />
<forms domain="domain.com" requireSSL="true" />
One other issue may be a connection of the session with the login, and if its lose the session is also lost the login.
In this case you must check the expire and recycle of the session. If its in memory consider to move it on database.
Also consider this question Can some hacker steal a web browser cookie from a user and login with that name on a web site? for more settings on the cookies.
Related answer Multiple applications using same login database logging each other out