Out of curiosity, where are you hosting?
The problem you reported was happening when RBE read the email to convert into a reply. Because you probably didn’t allow HTML messages, it was trying to convert the HTML email into plain text. That uses the HTML2Text library. The error message lead me to believe that HTML2Text didn’t get bundled with the plugin release, but it wasn’t the case when I checked the bundle itself.
But I’m glad it’s working now. I’ll close the ticket as resolved.