This one caught me out recently. Scenario:

All http://officecdn.microsoft.com is whitelisted and available, definitely no issue there.

• All other updates seem to come down fine.
• There are no ADR rules at all in place.
• Running a full sync produces the error below:

My client downloads updates through a proxy server and naturally the correct details for the proxy were in place on the Site System Settings under Site System Roles. Also the checkbox for Use a proxy server when synchronizing software updates was checked under the Proxy And Account Settings tab under the Software Update Point Properties role.

SOLUTION:

Despite the fact that ADRs were NOT created or used in any way, it transpired that the checkbox Use a proxy server when downloading content by using automatic deployment rules also had to be checked. Once this was selected, O365 updates started syncing.