@JSpiers667x is spot on.  Just got off a screen share session with the QBO Commerce team.  The rep has seen this thread and witnessed the blank dialog issue themselves.  Their team was online with two other customers with similar issues.  I was told they were requesting assist from the backend team, which is exactly what we need.

One other thing we tried was editing the account info of the Shopify Balance acct on the QBO Bank transactions page.  That successfully removed the connector and allowed us to set it up new.  Unfortunately the problem remained so removing the connector doesn't help - this is a backend issue between QBO and Shopify.

One more bit of info:  the 571 error pops up almost immediately, you just don't see it because of the blank dialog.  Just move the dialog before completing your auth process and you'll see it pop up.