Sometime the Exchange roll up update may break the OWA
How to fix,
Run the Exchange Management Shell
Remove-OwaVirtualDirectory “owa (Default Web Site)”
Then recreate the owa with
New-OwaVirtualDirectory -OwaVersion “Exchange2007″ -Name “owa (Default Web Site)”
***It may failed the first time to recreate the OWA, restart the IIS services and then try the command again.
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