Outlook Error: There is a problem with the proxy server's security certificate. Outlook is unable to connect to the proxy server (Error Code 80000000)

I had an interesting issue while working on one project. When customers working with Outlook, sometimes some Outlook clients received the following certificate error message:

 

There is a problem with the proxy server's security certificate.
Outlook is unable to connect to the proxy server (Error Code 80000000).

 

 

I want to share my solution on this issue.

All Outlook сlients were 2013/2016 with last updates.

 

This error had nothing to do with a proxy server.

 

All names for Internal and External URLs were set to a name that matched the digital certificate.

 

DNS was configured to direct all connections to the Exchange servers.

 

Proxy exclusion in place on all workstations to ensure data to the Exchange server was sent direct, not through the proxy.

 

The server was configured with MAPI over HTTPS enabled on the organization configuration and all Outlook clients were creating MAPI over HTTPS connections to the server.

 

After a long search and troubleshooting, I found on one of the servers the difference in the configuration.

 

The difference was in the InternalClientsRequireSSL:$false option in Outlook Anywhere configuration.

 

 

I changed the value to InternalClientsRequireSSL:$true

 

 

and "iisreset" on the server.

 

After changing InternalClientsRequreSsl to $true the issue did not occur.

 

 

 

Hope this post has been helpful and interesting.