Twitter API error 215

Today, we discovered one of my clients Twitter feeds became broken.

I have tried switching to using the new API 1.1, but get the following error:

{“errors”:[{“message”:”Bad Authentication data”,”…

How to test the twitter API locally?

I’m trying to write a web application that would use Twitter via OAuth.
I run my local server as ‘localhost’, so I need the callback URL to be something like http://localhost/something/ …