Just wanted to provide an update on this topic. I too am having issues with HTTP POST to an endpoint on my local network. I'm trying to program a simple on/off remote button for my Hyperion ambient lighting setup that is behind my TV. This is my flow...
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First I need to switch to an instance before I can send the on/off command. When I press button A on the M5StickC, it completely locks up the device during the HTTP POST command. I cannot even short press the power button to reset it. I have to hold down the power button for 6 seconds to fully power it off, then I can power it on again. Same exact thing with the M5StickC+. The M5Fire is a little better. It doesn't lock up, but I still can't get the HTTP POST to be a success or a fail. I would expect some type of failure if it didn't work properly... but instead I get nothing.
I'll continue to troubleshoot. It might be due to the IP:port in the HTTP request, as my test with https://httpbin.org/post works fine and it doesn't use a port. I'll let you know if I figure anything out.