nginx loadbalancer and OAuth
That's probably not in any way related to nginx. OAuth (1, not 2) requires a signing URL, which would be http://webservice:81 if you moved it to port 81. Make sure that your OAuth code knows the website is actually on port 80 and not 81.
Either update your client to say it's port 81 or tell the server it's on 80.
Replace 81 with your favorite port