Office Add-ins error: Cannot connect to catalog

MTSBob: We sorted this out today.

• When we were initially manually configuring "Block Web Add-ins" (DisableAllCatalogs) and "Block Office Add-ins" (DisableOMEXCatalogs), we were setting it in HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\WEF\TrustedCatalogs.
• When you configure those two settings in policy (User \ Administrative Templates \ Microsoft Office 2016 \ Security Settings \ Trusts Center \ Trusted Catalogs), it gets written to HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\WEF\TrustedCatalogs.
• Because we need to target this particular Office Add-in to a subset of users, we cannot rely on setting the Trusted Catalog Location policy items so we leveraged registry preferences in order to make use of https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/dn789189(v=ws.11).
  When we dropped those keys we matched our initial test configuration which was to write the Office Add-in settings to to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\WEF\TrustedCatalogs.  Turns out that gets ignored or overridden when something is set at the HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\WEF\TrustedCatalogs level.
• Once we realized this behavior we corrected the discrepancy and decided to rely solely on registry preferences, not policies, and the add-in started working again. 

So we shot ourselves in the foot but it took us a while to realize it.