We have a login re-direct issue. We have tested disabling plugins one by one and have had no result. When we disable the BB theme the issue goes away. We used Twenty Seventeen as the replacement. You can see the issue by going to the link below and attempting to purchase tickets while not logged in. https://prinz.org.nz/event/strategy-and-evaluation-for-pr-and-communication/#tribe-tickets
As you will see you get stuck at the login window but you are logging in – you can prove this by going back in the browser and refreshing the page.