Outlook Web App didn’t initialize. If the problem continues, please contact your helpdesk. Couldn’t find a base theme (folder name=base)
This error is caused, when the base theme for the OWA is missing in \ClientAccess\Owa\14.1.287.0\themes.
You solve this issue by running the following command in a Exchange Managment Shell:
“C:\Program Files\Microsoft\Exchange Server\V14\Bin\UpdateCas.ps1?