Outlook Web App didn’t initialize. If the problem continues, please contact your helpdesk.

PROBLEM:
Exception type:Microsoft.Exchange.Clients.Owa.Core.OwaThemeManagerInitializationException
Exception message: Couldn’t find a base theme (folder name=base)
Call stack
Microsoft.Exchange.Clients.Owa.Core.ThemeManager.LoadThemeFiles()
Microsoft.Exchange.Clients.Owa.Core.OwaApplicationBase.ExecuteApplicationStart(Objectsender, EventArgs e)

SOLUTION
run UPDATECAS.PS1 from ‘C:\Program Files\Microsoft\Exchange Server\V14\Bin 
with Exchange Management Shell