Hi NewCoder,
Yes,I did the same with you and add the subgrid to my account entity form. Then I got the same error with yours.

I pressed F12 and got the 400 error. I think this is caused by the fetchxml.
So I did something like the docs(docs.microsoft.com/.../troubleshoot-set-up-sharepoint-online says:
1. Create a solution and add SharePoint Document entity.
2.Export it and copy the content of <fetchxml> belonged to All SharePoint Document view and use it to replace my view's content of <fetchxml>.
3. Zip the folder and delete the solution created before.
4.Import the solution and it can work now.
