You have changed the path to the web application in the proxy (/ -> /webapp). This is generally a bad idea as it breaks a whole bunch of stuff that you then need to fix. In this case it is probably the cookie path that is your immediate problem. To fix that you need to use the proxy_cookie_path directive. Once you fix that expect to find other problems.
You'd actually be better off redeploying your web application as the ROOT web app so that your proxy_pass directive becomes proxy_Pass http://127.0.0.1:8080/;