The specified user or domain group was not found.

I started getting the error "The specified user or domain group was not found" after I restored a SP 2007 site to SP 2010 with claims authentication.

The event log had the above error followed by the following error:

Object Cache: The super reader account utilized by the cache does not have sufficient permissions to SharePoint databases.

To configure the account use the following command 'stsadm -o setproperty -propertyname portalsuperreaderaccount -propertyvalue account -url webappurl'. It should be configured to be an account that has Read access to the SharePoint databases.

Additional Data:

Current default super reader account: NT AUTHORITY\LOCAL SERVICE

To fix the error I had to run the following Power shell commands:

$wa = Get-SPWebApplication -Identity "http://yourwebapp"

$wa.Properties["portalsuperuseraccount"] = "domain\cacheuser"

$wa.Properties["portalsuperreaderaccount"] = "domain\cacheuser"

$wa.Update()