Outlook Web Access broke today after a Windows Update Rollup 4-v2 for Exchange Server 2010 SP1.
Trying to access OWA will get you the following error:
Outlook Web App didn’t initialize. If the problem continues, please contact your helpdesk.
Couldn’t find a base theme (folder name=base)
To fix it, open Exchange Management Shell and run the following powershell script:
C:\Program Files\Microsoft\Exchange Server\V14\Bin\UpdateCas.ps1