Exchange 2013 CU2 - Can't access ECP - Unexpected Error 500

I am running 7x Exchange 2013 CU2 servers (3x CAS in NLB, 4x Mailbox in DAGs) on Server 2008 R2 Enterprise, along with a 2010 environment. Recently, I have started getting '500 - Unexpected Errors' when accessing ECP for Exchange 2013. ECP works fine for Exchange 2010, and OWA works fine too.

The Application Event Log on my Exchange 2013 CU2 mailbox servers shows the following from the 'MSExchange Control Panel' source, with Event ID 4:

Current user: 'domain.internal/Microsoft Exchange System Objects/Monitoring Mailboxes/HealthMailbox673bae33faa54610a05da90c95912482'
Request for URL 'https://domain-ex13mb01.domain.internal:444/ecp/default.aspx(https://scn-ex13ca03/ecp/)' failed with the following error:
System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Exchange.Clients.Common.Canary15.Init(Byte[] userContextIdBinary, Byte[] timeStampBinary, String logonUniqueKey, Byte[] hashBinary, String logData)
   at Microsoft.Exchange.Clients.Common.Canary15..ctor(String logonUniqueKey)
   at Microsoft.Exchange.Clients.Common.Canary15Cookie.TryCreateFromHttpCookie(HttpCookie cookie, String logonUniqueKey, Canary15Profile profile)
   at Microsoft.Exchange.Clients.Common.Canary15Cookie.TryCreateFromHttpContext(HttpContext httpContext, String logOnUniqueKey, Canary15Profile profile)
   at Microsoft.Exchange.Management.ControlPanel.CanaryExtensions.CheckCanary15(HttpContext context, Boolean shouldRenew, String canaryName)
   at Microsoft.Exchange.Management.ControlPanel.CanaryExtensions.CheckCanary(HttpContext context)
   at Microsoft.Exchange.Management.ControlPanel.RbacModule.Application_PostAuthenticateRequest(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
   at Microsoft.Exchange.Clients.Common.Canary15.Init(Byte[] userContextIdBinary, Byte[] timeStampBinary, String logonUniqueKey, Byte[] hashBinary, String logData)
   at Microsoft.Exchange.Clients.Common.Canary15..ctor(String logonUniqueKey)
   at Microsoft.Exchange.Clients.Common.Canary15Cookie.TryCreateFromHttpCookie(HttpCookie cookie, String logonUniqueKey, Canary15Profile profile)
   at Microsoft.Exchange.Clients.Common.Canary15Cookie.TryCreateFromHttpContext(HttpContext httpContext, String logOnUniqueKey, Canary15Profile profile)
   at Microsoft.Exchange.Management.ControlPanel.CanaryExtensions.CheckCanary15(HttpContext context, Boolean shouldRenew, String canaryName)
   at Microsoft.Exchange.Management.ControlPanel.CanaryExtensions.CheckCanary(HttpContext context)
   at Microsoft.Exchange.Management.ControlPanel.RbacModule.Application_PostAuthenticateRequest(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Any ideas?

I have already tried restarting all of the servers in the environment, performing 'lodctr /r', etc.