I have created Sharepoint Claims site with SQL authentication(provided Username and Password). Claims user is able to access the site only after the Windows authentication(Site Collection Administrator) is done and till the cache gets clear. The below error is occuring over and again once the AppPool is reset or cache is cleared in the server. As per my observation the Site collection administrator has access to all the sharepoint databases and content DB. The SQL user has access only for Site Content DB and not for share point databases.
I believe if the site authentication is changed from SQL to windows, the problem should get resolved. Request you to let me know as to how to modify from SQL authentication to Windows authentication.