Issue with Webform_doPostBackWithOptions

We have a web page that allows a client to maintain client information within our system
The page makes a WebForm_doPostBackWithOptions javascript call to update the information - this page worked fine before we changed to the Haproxy and it works fine if we bypass the proxy and go to the webserver directly but when the page goes through the proxy the page just seems to hang. Other javascripts seem to run ok it appears to be just this one particular script that has the issue.

I’m not the webadmin, I’m a developer whose been tasked without trying to fix this. Has anyone else had an issue with this script or have any idea what might be the problem or the solution

Share the output of haproxy -vv, uname -a, the full configuration and the logs of the failing requests.