Further update - Problem appears to be fixed.
One difference in browser errors is the prepareFooter error:
Uncaught ReferenceError: highlight is not defined
at HTMLDocument.prepareFooter ((index):3508)
Further digging showed that the prepareFooter() is where custom javascript is placed and that was the issue. Custom java script was breaking the page during prepareFooter() which killed the reply functionality (along with a lot of other things).
The java script was there to add functionality to highlight.js for syntax highlighting. now to dig into what has changed with highlight.js in the latest version.
