Earlier today this error started to come up which I have not seen before since using Yahoo OAuth for several years now.
OAuth oauth_problem="OST_FORMAT_ERROR"
Any ideas on how to fix this? There is no documentation anywhere about it and a Google search comes up with almost nothing for OST_FORMAT_ERROR.
This was a regression in Yahoo's code. They fixed it a few hours later. It was only affecting clients using OAuth 1.0 (they are discontinuation support for it)
You should upgrade your code to use OAuth 2.0