Invalid API Key provided
Not so much an answer as a possible approach to debugging:

1) find out the URL that this command visits and see if you get the same result. If so it's a bug with authorize.net

2) if that URL works then it's a bug with civi. In this case get it to spit out what the api key is and check that against the vale you've entered, checking they're the same. 

The error should lie in one of those two places.