How to address Federation Trust issues in Hybrid Configuration Wizard (HCW)

johndennis250 , I am sorry, I didn't see the comment and I wasn't notified about it. I really hope you managed to find the issue / old proxy.

These would have been my suggestions:

1. Run this Exchange Management Shell on-premises: Get-ExchangeServer | FL identity, admindisplayversion, serverrole, *proxy*
2. Run in CMD netsh winhttp show proxy
3. Download PSExec tool on the Exchange server(s) – start with the server that threw the error  and run the following command PsExec.exe -i -s "c:\Program Files\Internet Explorer\iexplore.exe" to launch IE under System Account.

Now, in IE settings, check if any proxy is set there or if you have “Automatically Detect Settings” Enabled for the Local System Account combined with PAC file.
From on-premises Exchange to Office 365, the Exchange 2010 MBX & CAS or 2013 MBX (backend) or 2016 / 2019 would need outbound Internet access to the Microsoft Federation Gateway in addition to https://outlook.office365.com/ews/exchange.asmx

Verify the machine/system account can access these Microsoft Federation Gateway URLs:

https://nexus.microsoftonline-p.com/federationmetadata/2006-12/federationmetadata.xml  [<-- You should see an xml page.]

https://login.microsoftonline.com/extSTS.srf   [<-- You should see “Sorry, but we’re having trouble signing you in”.]

https://domains.live.com/service/managedelegation2.asmx   [<-- You should see the operations supported by ManageDelegation2.]

4. On the Exchange server(s) open regedit and see if a proxy is set there: HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ by looking at ProxyEnable if 1 and a ProxyServer is set.