Error Code: 504 Gateway Timeout only when using the HUD

[jamin...@gmail.com]

Configuration: 

- Win 10 (also tried Ubuntu 20.04) 

- ZAP 2.10.0

- Tried both Chrome and Firefox launched both with and without HUD from ZAP. 

I am doing some testing on a Bug Bounty site. I have two domains to test real.example.com and test.example.com. ZAP works perfectly on the test site but when attempting to load the real site I get a stacktrace (below) and a 504 Gateway Timeout Error in the history. 

Things I have tried: 

- Just browsing from a non-proxied browser - This works just fine

- Adjusting the User Agent - Does not fix the issue

- Using other tools - Other proxy tools work just fine. 

- Increasing the Connection Timeout Time - I went up to 120 seconds with no change

- Attempted both http and https - I was able to connect to both test and real sites when using HTTPS but ONLY if I did not use the HUD. 

Here is the stacktrace:

Any advice or further information that can assist in my search is greatly appreciated. 

Thanks