504 Gateway Timeout only when using the HUD

[contaminatedesert]

Describe the bug
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.

If I do not use the HUD it works fine, but I get a gateway timeout when using the HUD.

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 - Only HTTPS works and only when NOT using the HUD.

Software versions

• ZAP: 2.10.0
• OS: Windows 10, also tried in Ubuntu 20.04 with no change.
• Java: openjdk version 1.8.0_292
• Browser: Tried with both embedded browsers (Chrome and Firefox)

Would you like to help fix this issue?
I would like to use the HUD but the program is obviously still useable without it so yes. I would like assistance if possible.

[psiinon]

Does the site in question support HTTPS?
The HUD can only work over HTTPS as is uses modern technologies like WebSockets and Web Workers.
If a site doesnt support HTTPS then ZAP "upgrades" it to HTTPS internally (ie within ZAP).
This works for straightforward cases but it wouldnt surprise me if if failed for more unusual cases.
Unfortunately theres not enough info here for us to know whats going on.