Unexpected token < in JSON at position 0

I am creating a Christmas light show using Xlights and WLED.
I have just setup my show network using Falcon Pi Player (fpp) and a handful of ESP32s running WLED. I have configured fpp to route between my home network and my show network so that I can access each esp from my home network. However, when I search to connect to the esp, it loads the UI but then gives me this error:

unexpected token < in JSON at position 0
Connection to light failed!

If I connect my laptop to the show network directly, I can connect using the IP address mentioned above, but when I search it still comes back with the error.
When I use Xlights, it can see that WLED is online and I can upload configurations to the controller. But when I click ‘Open’ it searches the above URL and fails.

Does anyone know how to fix this issue?
Thank You!

My fpp exists on my home network as, and exists on my show network as (bridging the network connection).
The WLED controller exists on the show network at

You have to configure websockets on fpp as well or build version without websockets support.
Other than that UI was not designed to work across proxies or reverse-proxies.

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All sorted now. It turned out to be a mixup between communication between IP addresses and proxies.

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What did you do to resolve the problem .