My code worked great until one week ago when suddenly stopped working. This worked fine, and nothing changed in code.
Now, it throws OST_OAUTH_SIGNATURE_INVALID_ERROR. I can not find anything relevant why this is happening. I make a call to Contacts API and I have double checked signature is fine, as well as Consumer key and secret that I use.
I have found the problem in the implementation of OauthBase class in the way it generates signature base.
Of great help is OAuth Test Console, you can use it as reference to compare if your generated signature is good. Strangely enough, Yahoo! servers worked for a while with the bogus implementation as I mentioned before.
So, coming back to where there's the problem, if you are using current implementation there is a bug report on this issue.