OpenSSL::SSL::SSLError Exception: wrong version number whenever Pusher is invoked

I'm in the process of upgrading from rails 5.2.3 to 6.0.4. One of the pain points has been this error whenever a Pusher method is called, for example:

Pusher.trigger(channel, event, options)

Raises this error:

OpenSSL::SSL::SSLError:
       SSL_connect returned=1 errno=0 state=error: wrong version number

This issue didn't exist before, on Rails 5.2.3, so I'm stumped as to what's causing it now.

I'm using Pusher (2.0.1) and Ruby (2.6.2).

Any ideas?