something with the Oauth2 for Maniaplanet4 seems to be broken.
It worked fine a few days ago, I cannot tell exactly when it broke but now I'm suddenly getting either this error...

And that's with a request to http://www.maniaplanet.com/login/oauth2/access_token will all the parameters required according to this
Or, when it decides to actually grant me an access_token, I get a 403 Forbidden error for http://www.maniaplanet.com/webservices/me