Sharepoint users are logged out

Everyd开发者_开发知识库ay at sometime (mostly near the noon) all users of our sharepoint (2007) except "System Account" automatically get logged out and cannot login for about 30mins. I have no idea what possibly causes this situation. Can anyone help?

Thanks in advance.

Update: we use Active Directory for user authentication. At situations described above we can successfully login to Active Directory but it seems that SharePoint fails to login.

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When you say 'cannot login for about 30mins' do you mean that the site is inaccessible or users get an Access Denied page (and I mean the SharePoint one)? If the site is inaccessible then the SharePoint web server is probably down.

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On the 30 minutes what are the events in the Windows Logs of the server ?

What can be the cause : Maybe the primary DNS is filled in, and not the secondary. And maybe there is a reboot of the first DNS server ? Or maybe a network firewall or proxy is acting weird ?