Howdy Tess,
Sorry for the trouble here with activating Eventbrite Tickets!
I just tried activating and authorizing the plugin myself on a fresh test site and it seemed to work ok. I see that you also tried:
<ul class=”meta”>
<li class=”troubleshoot_plugins”>Deactivated all other plugins? Yes
<li class=”troubleshoot_theme”>Switched to a default theme? Yes
Did you happen to notice any difference when trying to authorize Eventbrite Tickets with your Eventbrite.com account when using a default WordPress theme (like Twenty Fifteen) and deactivating all other non-Modern Tribe plugins?
Also, I noticed you are using WordPress 4.3.1. While it’s a long shot that could be the issue, is there a reason for not updating to WordPress 4.4? It would be good to test with that as well.
Let’s start here and see what we find.
Cheers!
Geoff