Possible solutions:

1 - Force from CAS PowerShell: Set-CASMailbox 'user' ActiveSyncEnabled $True

2 - Delete and recreate the Activesync virtual directory by using the following commands:

Remove-ActiveSyncVirtualDirectory -Identity "Microsoft-Server-ActiveSync (Default Web Site)"

New-ActiveSyncVirtualDirectory -Server "xxx" -WebSiteName "Default Web Site" -ExternalURL "http://www.xxx.com/Microsoft-Server-ActiveSync"

http://technet.microsoft.com/en-us/library/aa997160(EXCHG.80).aspx

3 - This error could be caused by device ID restriction on the Activesync policy

Test if the deviceID restriction is null, if the list is NULL, which is the default, a user can sync with any device

Get-ActiveSyncDeviceStatistics -Mailbox:"<EmailAlias>" |fl DeviceID

Disable your Activesync policy for testing (wait some times for replica once policy has been removed)