My 2 cents on this:
Step 1: Go to the People page.
Step 2: Open Developer tools, Application and delete all Cookies for this site.
Step 3: Click the Sandbox link. The page will load with errors, but the + button will be visible.
Step 4: Click the + (Add) button.
Step 5: Login back to App Store Connect from a new Tab.
Step 6: Hit the save button on the first Tab after filling-in the details.
After I added the first tester the page loads fine for me.
Kaniosm
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I thought I was doing something wrong. Can't believe this is broken!