One of your customer's SharePoint 2007 portal is behaving in an usual way. When site collection administrators logs in we are able to successfully log in and access the home page but when any non-site collection administrator logs in he/she ends up with the below given error.
As an immediate troubleshooting points you can verify permissions of the effected user/group against the following:-
Homepage permissions - a) Validate if there are any unique permissions set for the homepage. b) Validate the homepage is not in a checked out or pending approval state.
Library permissions - Validate the permissions of the document library where the homepage is residing.
Any webpart/list view added in the homepage - Validate if there are any list added as webpart on the home page. If yes, validate the list permission.
Masterpage - Check for the status of the applied masterpage and ensure that it is not in a checkedout or pending approval state.
Additionally, there are times when you have Alternate Access Mappings configuration related problems due to which this error comes. Then there are has been scenarios where deactivating and activating the publishing feature fixed the issue. It is advised that you share the ULS logs, incase the above troubleshooting tips does not help you out.
Most likely, there is a web part on that page that references a list or library with custom permissions, and those users do not have permissions to see that list/library.
Have one of those users navigate to All Site Content(_layouts/viewlsts.aspx). Can they see the lists referenced in the home page?