Just an update, my app previews finished processing somewhere in the range of 7-11 hours from when I first posted them. I made no changes, I just waited.

My app previews are coming up on about 3 hours of processing time and are not complete. I have submitted previews in the past and they have worked. Is there a current issue on the App Store Connect server side? Thank you

I've been having this same problem (now iOS 17) but what ended up working for me was simply creating a new sandbox tester account (I had old ones setup from years ago I was trying to use.) All the old ones gave me the "Cannot Connect" error when trying to go into the 'Manage' screen even though they were initially able to authenticate. The new sandbox tester account let me into 'Manage'