Events+ Twitter authorization token is missing.

I have installed the Events+ plugin on my development site (http://dev.tsv.tweetup.billi.am) and I'm receiving an error when I try to log in with Twitter:

"There is no request token for this page. That's the special key we need from applications asking to use your Twitter account. Please go back to the site or application that sent you here and try again; it was probably just a mistake."

I've reset the keys multiple times and made sure all the application and plugin settings are correct as per the instructions and some random advice I've seen in threads on the WPMU community forums as to how the problem might be fixed. I'm all out of ideas.