Failing to Authorise, unhelpful error message

Hello,
In trying to authenticate with Eventbrite, it successfully gets to the App authorisation page, but upon clicking "Allow" it redirects back to the Wordpress side to an URL like (values stripped):

```
http://formalabs.org/wp-admin/tools.php?nonce=state=&code=&service=eventbrite&kr_nonce=&action=verify&page=keyring
```

..and displays the message `There was a problem authorizing with Eventbrite. Please try again in a moment.`.

I don't PHP. Any hints or ideas? :)