Hey Eric,
It seems like I still unable to replicate this even i open both browser on my laptop (chrome and Opera browser).
Maybe you can try this way :
1. If you facing this issue, try enable error reporting to maximum from backend
http://take.ms/u2fpf
2. Open your browser development console tool, you will see this ajax request when you press reply button.
http://take.ms/KpSmf
3. Then click inside this ajax request, and double check whether this ajax call namespace is called 'site/controllers/conversations/reply'
http://take.ms/aN7CN
4. If yes, then click on that `response` tab, it might be show the error message.
http://take.ms/Efpjow
Keep us update regarding this.