Update: as of about 2024-04-29T20:30:00 (UTC) this just started working again for us. Seems like MS fixed whatever broke before we could get to the bottom of it. We had started entering the ticket and in reproducing the steps noticed it had started working. It was definitely still broken as of 2-3 hours ago for our tenant though.
Short version: Using the External Web Page Container in a customer page with a URL that points to our SharePoint site. Now we get a page with the Microsoft 365 logo, some text that says /Please sign in to view this file/, and a blue button labeled /Sign in/. Clicking the button causes an auth window to popup and then go away (I am already logged in to CRM online after all) but the page remains the same. Looking at the logs in the F12 developer tools shows that a POST request is made to our SharePoint online tenant to /_api/contextinfo as well as second request to /_api/web/Language - both fail with a 401 unauthorized. I can click the button over and over, never getting authorized. I have tried it in Chrome & Edge and get the error, though oddly enough Firefox seems to work, though it took a couple of refreshes. I also tried turning off Tracking Prevention in Edge which did not resolve the problem. This happens with embedded content from a SharePoint library as well as OneDrive.
I am not quite sure if it is a browser issue, a Microsoft servers issue, or possibly a configuration issue. Also, there may be a better way to surface this info (it is currently an Excel sheet with some helpful sales info) so I am open to alternative approaches as well. Here is a screenshot of the Page (with domain obscured) for reference:
When opening it in a fresh Firefox instance, it does prompt me to allow cookies between the SharePoint and Dynamics domains:
and then if I allow them, sign in, then refresh a couple of times it will finally load:
And for additional reference, here is the notice in the Issues tab of the F12 developer tools console regarding cookies:
and here are the settings I tried in Edge:
It is also worth noting that the SharePoint Site Settings -> HTML Field Security settings include all the various (powerapps.com, dynamics.com, etc.) domains that would be required:
