I thought I was doing something wrong. Can't believe this is broken!

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.