App Engine Returning Error 500 on Post Requests from
If you're not seeing anything in your server logs about the error, that suggests to me that you might have a configuration error in one of your .yaml files. Are GET requests working? Are you sure that you are sending your POST requests to an endpoint that is handled by your application? Check for typos in your JavaScript and application Route definitions, and check for a catch-all request handler (e.g. /*) that might be receiving the request and failing to respond. 
Sharing the contents of your app.yaml, your server-side URL routes, and a snippet of your JavaScript would really help us to help you.
I am getting error 500 on every second POST request made from a browser (chrome and firefox) irrespective of whether it is a Jquery Post or Form submissions, app engine is alternating between error 500, and successful post. The error 500 are not appearing anyway in the logs. 
I have tested this with over 5 different post handlers, the errors are only occurring on production not on the Local SDK server.
Note that the requests are perfectly successful when made from a python script using the requests module.