I was getting it too. I tried a bunch of stuff- deleting app, reinviting testing user, deleting and re-installing TestFlight. Nothing has worked. Now, I don't get an error, but the latest version of my app in TestFlight won't update. It acts like it's going to, but it fails and I get the "install" but reappearing. I got the "Beta Expired" and "Integrity could not be verified" errors along the way today.

I've been wondering the same thing. I agree with you, if Apple is collected sales tax from the end user, then they are responsible for remitting to the applicable state where the purchase was made. Did you every find anything out about this?