Hi,
You have found it but a technique you could use to find this issue is something most GDC developers will be familiar with, but is not as well publicised for the Web clients is to view the AUI Tree.
To do this with 2.50 you need to ensure that the proxy is started with the --development option
https://4js.com/online_documentation/fjs-gas-2.50.00-manual-html/?path=fjs-gas-2.50.00-manual#c_gas_tools_html5proxy.html. This can be achieved by editing the appropriate proxy param in as.xcf ...
<RESOURCE Id="res.gwcproxy.param" Source="INTERNAL">--development</RESOURCE>
<RESOURCE Id="res.html5proxy.param" Source="INTERNAL">--development</RESOURCE>
When this runs, you will see a few extra icons at the top, hover over them to see the name, the one third from left should be "Display AUI Tree". This will open the AUI Tree in another web page and from that you can find the ActionDefault nodes to see what file was being loaded etc.
Reuben