From the Google Adwords API Blog

<quote1>
Yesterday we identified a bug in the AdWords API registration process that is preventing some developers from successfully entering their billing information and accepting/rejecting the Terms and Conditions. We have identified the issue and hope to have a fix in place in the next few days.
</quote1>
Link:
adwordsapi.blogspot.com/2006/0 ...

<quote2>
Yesterday we identified a bug in the AdWords API sandbox that results in requests returning error code 131. We are working on a fix now. In the meantime, we would like to recommend a work-around.

Developers can still work with the sandbox by using the older V4 request headers. Specifically, developers should use “token” instead of “developerToken,” and they should not include “applicationToken” in their headers.
</quote2>
Link:
adwordsapi.blogspot.com/2006/0 ...

One bug fixed today

Link:
adwordsapi.blogspot.com/2006/0 ...