I had this problem a few months back, so I apologize if this looks like a duplicate post.
I’ve created a page include that works just fine for Internal users but when logged in as a Partner User, the page include hangs with the “loading…” message on the screen.
Visualforce code for Parent Page (CaseWall):
Visualforce code for Embedded Page (CaseDetail):
I’ve confirmed that all of the security settings are correct for the Visualforce pages. Partner users have permission to view these pages.
The only clue that I am receiving as to what is wrong is that when I disable all of the Models then the page loads correctly (it doesn’t have any of the field data, but at least the page loads).
Here is what it looks like with the “Load Model data on page load” box unchecked on all of the Models: