Haproxy showing 502 for backend but can get to the backend directly?

We have an haproxy talking to a virtualized IIS backend. Intermittently (~15 minutes) haproxy starts getting 502’s for the backend and websites stop working. However going straight to the backend there is no issue getting the websites to pull up. In addition an iisrest nor most of the time does a haproxy restart resolve the issue. The only resolution is to reset the backend adapter card or STOP IIS manually and restart IIS.

When the system stops working through haproxy and throwing 502 the backend does NOT show down in the haproxy logs but obviously does show down and back up when we restart the adapter or stop and start IIS.

This just started happening last week and is happening on multiple backend servers on multiple version of windows server (2016,2019,2022).

Any insight into why haproxy would throw a 502 when going directly to to the backend would be working in addition to why most of the time restarting haproxy does not resolve this issue would be greatly appreciated.

I have almost same 2 same issue but not with IIS but with Apache, I have partially solve the same, but using server and client timeout in both front and back end in haproxy