Lync client request:

- <SetForwarding xmlns="https://www.ecma-international.org/standards/ecma-323/csta/ed3">
<device>tel:1799;phone-context=enterprise</device>
<forwardingType>forwardImmediate</forwardingType>
<activateForward>false</activateForward>
</SetForwarding>

CSTA gateway response:

- <SetForwardingResponse xmlns="https://www.ecma-international.org/standards/ecma-323/csta/ed3">
</SetForwardingResponse>

CSTA gateway response:

- <ForwardingEvent xmlns="https://www.ecma-international.org/standards/ecma-323/csta/ed3">
<monitorCrossRefID>9a19c24c</monitorCrossRefID>
+  <device AA="https://www.w3.org/2001/XMLSchema-instance"
type="SubjectDeviceID">
<forwardStatus>true</forwardStatus>
<forwardTo typeOfNumber="dialingNumber"
bitRate="constant">tel:;phone-context=enterprise</forwardTo>
</ForwardingEvent>

In this case CSTA gateway sent event for forwarding highlighted at yellow above. In non-working scenario I have been working this event was never sent so reason why Lync client crashes. This means that this problem can be avoided if CSTA gateway is compliant and send expected events.

Even after potential correction is released Lync client will display a pop-up asking users if they would like to keep trying applying forwarding as event was not received. So basically potential hot fix will only avoid access violation and CSTA gateway behavior should be fixed.

For this specific scenario adding registry key to disable RCC Forwarding was an effective workaround:

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9. Disable RCC Forwarding (DisableRCCForwarding)

Remote call control is a feature of a PBX that allows the VoIP phones on a network to come online and share the same configuration that is configured for them on the PBX that hosts the VoIP network. One of the features that is shared by most PBXs is call forwarding. Call forwarding allows VoIP phones to forward calls to destinations that are chosen by the VoIP phones user. When a Communicator client is configured as a remote call control client, it cannot inherit the call forwarding features that are designed into the Communications Server Enterprise Voice client. Instead it will inherit the call forwarding features that are provided by the PBX that is hosting it on the VoIP network. Third-party remote call control call forwarding is not a fully supported feature. However, it is recognized as a compatibility issue that appears intermittently when the Communicator client is brought into an existing VoIP network as a remote call control client with a legacy PBX installation. As a means to troubleshoot issues with remote call control call forwarding, the Disable RCC Forwarding group policy was implemented. This allows administrators to toggle the remote call control call forwarding feature on or off while troubleshooting remote call control call forwarding issues.

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 This registry key should be created in the following path:

HKLM\Software\Policies\Microsoft\Office\15.0\Lync

1.     On the Edit menu, point to New, and then click DWORD Value.
2.     Type DisableRCCForwarding, and then press ENTER.
3.     Right-click DisableRCCForwarding, and then click Modify.
4.     In the Value data box, type 1, and then click OK.
5.     Make sure this key does not exist on the HKCU tree on the same machine.
6.     Exit Registry Editor. 

As soon potential fix is released this post will be updated.

Looking into Event Viewer ‘Microsoft Office Alerts’ the following event was being generated every time problem was reproduced:

Microsoft Lync

CLSID: {00000000-0000-0000-0000-000000000000} CATID: {9B179D6E-9BDB-454b-BE3D-89F9A792BD39} The ActiveX Compatibility setting disabled loading this object to help protect your security.

1073695582

15.0.4433.1506

0x80004005 

Take  note of CLSID identifier above and open registry editor:

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\ActiveX Compatibility\{00000000-0000-0000-0000-000000000000} (if using LYNC 2013 32 bits on Windows 8 64 bits)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{00000000-0000-0000-0000-000000000000} (if running LYNC 2013 64 bits or running LYNC 2013 32 bits on Windows 8 32 bits)

You can remove this entry or change Compatibility Flags as described on the following KB articles:

https://support.microsoft.com/kb/2252664/en-us - Security Settings for ActiveX controls and OLE objects in Office 2003 and in the 2007 Office suite

https://support.microsoft.com/kb/909738/en-us - A Web page that contains a custom ActiveX control may not load as expected in Internet Explorer due to defense in depth changes introduced in cumulative security update 896688 (MS05-052)

https://support.microsoft.com/kb/240797/en-us - How to stop an ActiveX control from running in Internet Explorer

1. Lync 2013 Front End Service RTCSRV failing to start 
2. HTTPs connectivity failures reported in the event viewer [we will add some events here for reference:

Event 30988, Ls User Services

Sending HTTP request failed. Server functionality will be affected if messages are failing consistently.

Sending the message to https://URL.contoso.com:444/LiveServer/Replication failed. IP Address is 192.168.0.1. Error code is 2EFE. Content-Type is application/replication+xml. Http Error Code is 0.
Cause: Network connectivity issues or an incorrectly configured certificate on the destination server. Check the eventlog description for more information.
Resolution:
Check the destination server to see that it is listening on the same URI and it has certificate configured for MTLS. Other reasons might be network connectivity issues between the two servers.

Event 32178, LS User Services

Failed to sync data for Routing group {EB10E520-9B20-575D-9D4C-C06E5A937F65} from backup store.
Cause: This may indicate a problem with connectivity to backup database or some unknown product issue.
Resolution:
Ensure that connectivity to backup database is proper. If the error persists, please contact product support with server traces.

Event 32174, LS User Services

Server startup is being delayed because fabric pool manager has not finished initial placement of users.

Currently waiting for routing group: {EB10E520-9B20-575D-9D4C-C06E5A937F65}.
Number of groups potentially not yet placed: 1.
Total number of groups: 1.
Cause: This is normal during cold-start of a Pool and during server startup.
If you continue to see this message many times, it indicates that insufficient number of Front-Ends are available in the Pool.
Resolution:
During a cold-start of a large Pool it can take upto an hour for the placement process to finish as it needs to populate all the Front-End databases with data from the Backup Store. If the Pool is running and the Front-End is just started, this is normal for some time. If this repeats for a long time, ensure that all the Front-Ends configured for this Pool are up and running. If multiple Front-Ends have been recently decommissioned, run Reset-CsPoolRegistrarState -ResetType QuorumLossRecovery to enable the Pool to recover from Quorum Loss and make progress.

Cause:

You are likely to hit one of these issues if you have deployed non self-signed certificates into Trusted Root Certification Authorities instead of Intermediate Certification Authorities. This is a misconfiguration and can cause HTTP communication between Lync servers to be broken with untrusted root cert error. In Windows 2012 there is a high level of trust check for certification authentication, and hence this issue is exposed only for Lync deployments on Windows 2012.

Resolution:

You can follow the following steps to fix such misconfigurations: 
1. If you are using group policies to deploy certs (https://technet.microsoft.com/en-us/library/cc738131(v=WS.10).aspx) ensure Trusted Root Certification Authorities only contains self-signed certificates (where Issued To = Issued By). Move any non-self-signed certificate present in this store to Intermediate Certification Authorities 
2. If you are importing any new certificates (either on your DC or Windows 2012 machines), then ensure as part of import you choose Trusted Root Certification Authorities for any self-signed certificates and Intermediate Certification Authorities for any non-self-signed ones

UPDATE:

Public KB article has just been published at https://support.microsoft.com/kb/2795828 -
Lync Server 2013 Front-End service cannot start in Windows Server 2012