Hey, I moved your reply here to its own thread so we can try to find out whats the problem. I just gave it a quick try and the replay buffer works as expected for me.
You mentioned record from buffer works, but the "save replay buffer" hotkey does not save any file I assume?
To make sure, in the broadcast settings a path for the replay buffer is set and you increased the buffer from 1 second to something longer? And you setup the mentioned hotkey to save the last seconds to disk?
If you dont mind, give it a quick try on your desktop, without running any game. Just start a buffer, record a few seconds and then use the save replay buffer hotkey, see if a file is created. Some games can block hotkeys, so that would be a small test to make sure that this is not the problem.