Demystifying Hybrid Free/Busy: Finding errors and troubleshooting

JeremyTBradshaw , I would be surprised if the issue exists for having TLS1.0 and 1.1 enabled. Negotiate should start by the highest available protocol and go down. 

Not sure if you are aware, but the process to enable TLS1.2 is a bit different since Windows Server 2012 R2.

In Windows 2012 R2 and above, TLS 1.2 is enabled by default in WinHTTP component BUT in Windows 2012 (not R2) or older it is not enabled by default, using TLS1.0 and SSL3

For Windows server 2012 you need to install KB3140245 and add a registry key to allow the use of TLS 1.2.

I believe a Dword,  DefaultSecureProtocols should exist in the following paths: 

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp

The value to use TLS 1.2 should be 0x00000800 (2048 dec) . If you want to enabled TLS 1.2, TLS 1.1 and TLS 1.0 the value should be 0x00000A80 (2688 dec)

You can find the official documentation in:

Update to enable TLS 1.1 and TLS 1.2 as default secure protocols in WinHTTP in Windows

https://support.microsoft.com/en-us/topic/update-to-enable-tls-1-1-and-tls-1-2-as-default-secure-protocols-in-winhttp-in-windows-c4bd73d2-31d7-761e-0178-11268bb10392

Another aspect that may afect in Exchange pre 2016 is the InternetWebProxyBypassList property, in Exchage 2010 and 2013 it cannot be set.

I would be very interested in knowing if you do get it to work in Exchange 2010. Also, from my undesratnding the HCW does not configure OAuth in coexistence with Exchande 2010.

Configure OAuth authentication between Exchange and Exchange Online | Microsoft Learn

"The Hybrid Configuration wizard automatically configures OAuth authentication between Exchange 2013 and Exchange Online organizations. If your Exchange 2013 organization contains Exchange 2010 or Exchange 2007 servers, the Hybrid Configuration wizard doesn't configure OAuth authentication between the on-premises and online Exchange organizations. These deployments continue to use the federation trust process by default. "