Exchange Server Side Sync Works with CRM 2013, but not with 2015, although mailbox test succeeds

We are trying out CRM 2015 and cannot get Server Side Sync to work with Exchange 2010. We have tried the same working configuration as in CRM 2013, but it fails. I have been configuring Server Side Sync with Exchange in CRM 2013 without any issues and have been troubleshooting various errors, but just can't get a victory on CRM 2015.

When Enabling & Testing mailbox - message get delivered, everything OK, alerts say that sucessfully configured. When sending out mail via Email activity - stuck on Pending Send and 0 delivery attempts.

In tracing, I have exception:

>Crm Exception: Message: The email that you are trying to deliver cannot be accepted by Microsoft Dynamics CRM. Reason Code: NoCorrelationMatch., ErrorCode: -2147098101, InnerException: Microsoft.Crm.CrmException: The email that you are trying to deliver cannot be accepted by Microsoft Dynamics CRM. Reason Code: NoCorrelationMatch.
   at Microsoft.Crm.Common.ObjectModel.EmailService.CheckForEmailAcceptance(String subject, ExecutionContext context, TrackingManager trackingManager, AddressManager addressManager, AddressEntry[][] allAddressEntries, Entity extraProperties)
   at Microsoft.Crm.Common.ObjectModel.EmailService.Deliver(Boolean userPromote, Guid emailId, String messageId, String subject, String from, String to, String cc, String bcc, DateTime receivedOn, String submittedBy, String importance, String body, BusinessEntityCollection attachments, Guid campaignResponseId, Entity emailDeltaEntity, ExecutionContext context, Boolean validateBeforeDeliver)
   at Microsoft.Crm.Common.ObjectModel.EmailService.DeliverIncoming(String messageId, String subject, String from, String to, String cc, String bcc, DateTime receivedOn, String submittedBy, String importance, String body, BusinessEntityCollection attachments, Entity extraProperties, Boolean validateBeforeCreate, ExecutionContext context)

Process Email Setting is set to Server Side. However there is an exception in CrmAsyncService log when I save these settings:

Process:CrmAsyncService |Organization:3b4c87ed-3fa5-e411-80ba-005056bd50b8 |Thread:   12(MSCRM:-NotificationManager.NotificationWatcherThreadProc) |Category: Exception |User: 00000000-0000-0000-0000-000000000000 |Level: Error |ReqId: 00000000-0000-0000-0000-000000000000 | NotificationManager.InvokeEventHandlers  ilOffset = 0x6E
	at NotificationManager.InvokeEventHandlers(INotificationsProvider currentProvider, NotificationEvent notification, List`1 eventHandlers)  ilOffset = 0x6E
	at NotificationManager.NotificationWatcherThreadProc(Object locatorServiceContext)  ilOffset = 0xD9
	at ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)  ilOffset = 0x70
	at ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)  ilOffset = 0x4
	at ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)  ilOffset = 0x2F
	at ThreadHelper.ThreadStart(Object obj)  ilOffset = 0x20
>delegate Microsoft.Crm.Asynchronous.OrgServerSideSyncSettings:UpdateOrgSettingsLight for eventId netSettingsOrganization threw an exception: System.InvalidCastException: Unable to cast object of type 'System.Collections.Concurrent.ConcurrentDictionary`2[System.Guid,Microsoft.Crm.ObjectModel.MailboxUtil+ServerSideSyncOrgSettings]' to type 'System.Collections.Generic.IDictionary`2[System.Guid,Microsoft.Crm.Caching.EmailConnectionChannelType]'.
   at Microsoft.Crm.Asynchronous.OrgServerSideSyncSettings.RemoveOrgFromOrgSettingsLightCache(Guid organizationId)
   at Microsoft.Crm.Asynchronous.OrgServerSideSyncSettings.UpdateOrgSettingsLight(NotificationEventArgs eventArgs)
   at Microsoft.Crm.NotificationManager.InvokeEventHandlers(INotificationsProvider currentProvider, NotificationEvent notification, List`1 eventHandlers)

Searching for ths exception, 0 results come up on the internet. (This post will be the first one to respond to the Search query :))

I may note that I am using Impersonation to check email accounts with service account provided in email server profile. Impersonation permissions are set up and this very Exchange for these same users can sync on CRM 2013.

I have tried changing CRM tracking token, providing User credentials for mailbox, setting User permissions to sysadmin and whatnot to help send out mails. But no success.

This appears to be bug or what? Can someone please suggest what additional logs/places I can view to debug this issue?