Failed "Setting up ACS Proxy" step when implementing hybrid search

Hi,

Here's my situation:

I have successfully implemented hybrid search between SharePoint 2016 and SharePoint Online on a development environment by following the steps in this article (https://technet.microsoft.com/en-us/library/dn720906(v=office.16).aspx). 

When repeating the same steps in production, the Onboard-CloudHybridSearch.ps1 script fails with the following error:

New-SPAzureAccessControlServiceApplicationProxy : The underlying connection was closed: Could not establish trust
relationship for the SSL/TLS secure channel.
At C:\Temp\Onboard-CloudHybridSearch.ps1:82 char:21+ ... $acsProxy = New-SPAzureAccessControlServiceApplicationProxy -Name $AC ...+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : InvalidData: (Microsoft.Share...pplicationProxy:SPCmdletNewAzur...pplicationProxy) [New-
   SPAzureAcce...pplicationProxy], WebException+ FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletNewAzureAccessControlServiceApplicationProxy

C:\Temp\Onboard-CloudHybridSearch.ps1 : ACS proxy '' is not set as the default. Configure manually through Service
Application Associations admin UI and retry.
At line:1 char:1
+ .\Onboard-CloudHybridSearch.ps1 -CloudSSAId "SPP16 Cloud Search Servi ...+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : OperationStopped: (ACS proxy '' is...n UI and retry.:String) [Write-Error], RuntimeExcep
   tion+ FullyQualifiedErrorId : ACS proxy '' is not set as the default. Configure manually through Service Application A
   ssociations admin UI and retry.,Onboard-CloudHybridSearch.ps1

The development and production environments are configured the same with the same certificates installed and the same firewall profiles.

I would really appreciate anyone pointing me in the right direction.

Thank you.