Exchange 2007 and Exchange Online Free/Busy Issue

I can't sing higher praises on the Exchange Hybrid Free/Busy Troubleshooter - available here https://support.microsoft.com/en-au/help/10092/troubleshooting-free-busy-issues-in-exchange-hybrid-environment

It got me out of a jam that was especially perplexing.

So I have an Exchange 2007 environment, and the customer would want to move all mailboxes to Exchange Online, but they want to deploy Exchange Hybrid because "Big Bang" doesn't work for them.

So I put in an Exchange 2013 "hybrid" server and ran HCW. All went fine except when an Ex07 user tries to query free/busy for a cloud/EXO user, the dreaded grey bar "No Information" is shown.

FIX: Exchange 2013's EWS InternalURL is set to the wrong URL - in my case, it was pointing to the UAG 2010's listener, which still publishes OWA 2007. So I did the following to fix it:

• Update Exchange 2013 EWS InternalURL to internal FQDN (so the Exchange 2007 can locate it) - EAC > Servers > Exchange 2013 Server > EWS (Default Web Site) > Internal URL Then perform IIS reset
• Removed the availability space object - Remove-AvailabilityAddressSpace -Identity ‘tenantname.mail.onmicrosoft.com' 
• Recreate the availability space object - Add-AvailabilityAddressSpace -ForestName tenantname.mail.onmicrosoft.com' -AccessMethod 'InternalProxy' -UseServiceAccount $true -ProxyUrl https://exchange2013internalFQDN.company.com/ews/exchange.asmx