I'd research prior discussions like this.
1. https://www.google.com/search?q=Call+stack+Microsoft.Exchange.Data.Directory.SystemConfiguration.AddressBookBase.GetAllRoomsAddressList%28ClientSecurityContext+clientSecurityContext%2C+A&ie=utf-8&oe=utf-8
2. Lead me to https://social.technet.microsoft.com/Forums/exchange/en-US/8ae2da35-1fb7-4969-a6f2-b108a2988612/owa-addressbook-error which claims they fixed it at...
3. http://blogs.msdn.com/b/dgoldman/archive/2007/12/05/deleting-the-all-rooms-address-list-stops-it-from-showing-up-in-owa.aspx
I no longer have access to OWA but hope sharing how to research such issues saves you a call to Microsoft.
Bob
I'm having trouble accessing our address book when composing new emails in OWA.
The following is the error I get when I click on the "TO:" field to bring up the address book
OWA version: 14.1.438.0
Mailbox server: SERVER.jpc.local
Exception
Exception type: System.NullReferenceException
Exception message: Object reference not set to an instance of an object.
Call stack
Microsoft.Exchange.Data.Directory.SystemConfiguration.AddressBookBase.GetAllRoomsAddressList(ClientSecurityContext clientSecurityContext, ADSystemConfigurationSession configurationSession)
Microsoft.Exchange.Clients.Owa.Core.UserContext.get_AllRoomsAddressList()
Microsoft.Exchange.Clients.Owa.Core.Directory.DirectoryAssistance.GetAllRoomsAddressBookInfo(UserContext userContext)
Microsoft.Exchange.Clients.Owa.Core.UserContext.get_AllRoomsAddressBookInfo()
Microsoft.Exchange.Clients.Owa.Premium.Controls.SecondaryNavigationDirectoryList.CreateCondensedDirectoryList(UserContext userContext, Boolean isRoomPicker)
Microsoft.Exchange.Clients.Owa.Premium.DirectoryView.RenderSecondaryNavigation(TextWriter output, UserContext userContext, Boolean isRoomPicker)
Microsoft.Exchange.Clients.Owa.Premium.Controls.NavigationHost.RenderSecondaryNavigation(TextWriter output, Boolean showContacts)
ASP.forms_premium_addressbook_aspx.__Render__control1(HtmlTextWriter __w, Control parameterContainer)
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
System.Web.UI.Page.Render(HtmlTextWriter writer)
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

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