Images in Outlook display red X

This is not confirmed, but would appear that an in-place upgrade to Windows 10 20H2 checks the box in Internet Option to "Do not save encrypted pages to disk".  

GPO Fix: Either user or computer policy (wherever you have browser settings configured)

Policies > Administrative Templates > Windows Components > Internet Explorer > Internet Control Panel > Advanced Page

Disable "Do not save encrypted pages to disk"

Local Fix:

Open Internet Options and click on the Advance Tab

Uncheck "Do not save encrypted pages to disk"

Source: HTTPS linked images in HTML emails display the red X - Outlook | Microsoft Docs