One thing you could try is the "haxe.enableCompilationServer": false - that can currently cause issues with some Firewall configurations, but should be fixed in the next release of the Haxe extension.
However, if openfl test html5 is already not working from the command line, this is unlikely to be a VSCode-specific problem.
I tried adding the enablecompilationserver to bother the user & workspace settings but no change.
As Flash builds from the command line there’s obviously something wrong with HTML5 - i get this error:
Error: EINVAL, invalid argument
at new Socket (net.js:156:18)
at process.stdin (node.js:664:19)
at Object. (C:\HaxeToolkit\haxe\lib\lime\6,2,0\templates\bin\node\http-server\bin\http-server:161:19)
at Module._compile (module.js:456:26)
at Object.Module._extensions…js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Function.Module.runMain (module.js:497:10)
at startup (node.js:119:16)
at node.js:906:3