Exchange 2013 missing dependencies

Question

After installing KB2880833, ecp and owa seem to be broken. When visiting the ecp Web site, it shows the following error:

Could not load file or assembly 'Microsoft.Exchange.Security' or one of its dependencies. The system cannot find the file specified.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.Exchange.Security' or one of its dependencies. The system cannot find the file specified.

This error is displayed even before loading the login page. The file Microsoft.Exchange.Security.dll does exist in the Bin folder of the Exchange installation. When visiting the owa Web site, you do get redirected to the login page, but after logging in you'll get redirected to a 500-error page.

In both the owa and ecp folder, there is a new web.config file. However, even when using the old web.config file, the error persists.

We can still access e-mail through Outlook, no mailboxes seem to have been affected. The problem seems to be with ecp and owa.

Edit - Looks like Exchange's Powershell is also missing a few dependencies, as the following had been logged to the event log:

Event code: 3008 
Event message: A configuration error has occurred. 

Application information: 
    Application domain: /LM/W3SVC/2/ROOT/PowerShell-24-130376967314709092 
    Trust level: Full 
    Application Virtual Path: /PowerShell 
    Application Path: C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\PowerShell-Proxy\ 
    Machine name: EXCH01 

Exception information: 
    Exception type: ConfigurationErrorsException 
    Exception message: Could not load file or assembly 'Microsoft.Exchange.Configuration.Core, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified. (C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\PowerShell-Proxy\web.config line 54)
   at ...

Could not load file or assembly 'Microsoft.Exchange.Configuration.Core, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified. (C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\PowerShell-Proxy\web.config line 54)
   at ...

Could not load file or assembly 'Microsoft.Exchange.Configuration.Core, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.
   at ...

Request information: 
    Request URL: https://localhost:444/powershell?clientApplication=ActiveMonitor 
    Request path: /powershell 
    User host address: ::1 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: NT AUTHORITY\SYSTEM 

Thread information: 
    Thread ID: 7 
    Thread account name: NT AUTHORITY\SYSTEM 
    Is impersonating: False 
    Stack trace:    at System.Web.Configuration.ConfigUtil.GetType(String typeName, String propertyName, ConfigurationElement configElement, XmlNode node, Boolean checkAptcaBit, Boolean ignoreCase)
   at System.Web.Configuration.Common.ModulesEntry.SecureGetType(String typeName, String propertyName, ConfigurationElement configElement)
   at System.Web.Configuration.Common.ModulesEntry..ctor(String name, String typeName, String propertyName, ConfigurationElement configElement)
   at System.Web.HttpApplication.BuildIntegratedModuleCollection(List`1 moduleList)
   at System.Web.HttpApplication.GetModuleCollection(IntPtr appContext)
   at System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers)
   at System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context)
   at System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context)
   at System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext)

Solution

Using a self-signed certificate on the Exchange server? I was and had the same problem. Uninstalling "Security Update for Exchange Server 2013 Cumulative Update 3 (KB2880833)" which got installed during regular maintenance last night fixed OWA before the uninstall dialog on the server was even gone.