ParaView Web and trame application usually only serve static content and everything else go through websocket. (The post request can be ignored as it is for session launching when a launcher is available)
Since Visualizer is old, the auto detection of protocol has a bug that was fixed maybe a year or so ago for trame. Because of that bug, you will need to manually provide the ws path in the url.
So for example if you were reaching the application with the following url https://my.domain.com/visualizer/ you should add a query to specify the ws connection.
https://my.domain.com/visualizer/?sessionURL=wss://my.domain.com/visualizer/ws
HTH,
Seb