Have you added this value "ExcludeExplicitO365Endpoint" to the registry on the affected Outlook clients?
If haven't yet, please try adding it and see if it can help with this issue.
The path in registry is HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\AutoDiscover (if the Autodiscover key doesn't exist, you can manually create it)
Please create a DWORD value named ExcludeExplicitO365Endpoint and set its value to 1.
Then restart Outlook and see if it can connect successfully
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