This error is often thrown when multiplexing is set as the default providers are set to multiplexing, which is not supported within an Azure environment. Often changing the default providers results in the ability to access and log into the site. Below is an example of the settings within the web.config which have been associated with this error.
<membership defaultProvider="MultiplexingMembershipProvider" userIsOnlineTimeWindow="10" hashAlgorithmType="HMACSHA512">
...
<roleManager enabled="true" defaultProvider="MultiplexingRoleProvider" cacheRolesInCookie="true">