This is going to sound a little presumptuous, but it looks like something Google is doing changed last night and broke their code library for their API.
New release: Binary only | Source will be updated soon
What happened?
In the gauthrequest.cs file, provided by Google, lines 408-9 look like this
// check the content type, it must be text
if (!response.ContentType.Equals(HttpFormPost.ContentType))
and this was working until last night. In this case HttpFormPost.ContentType = “text/plain”. Problem appears that now, the response.ContentType of the authrequest is being returned as “text/plain; charset=utf-8″. So that .Equals conditional no longer does the right thing, thus the authentication breaks.
To fix, I change line 409 to
...ContentType.StartsWith(HttpFormPost...
Now all is well again. Re-install and give it a go. I guess this is going to be standard for interfacing with a “beta” Google API. And we’ll just have to get used to it…