I’m seeing this as well. I can’t create replies on either iOS or Mac Safari but Chrome works fine on both platforms.
Thank you for the tip. I could install an old version of google chrome (63). Currently google chrome requires IOS 10. It is bad for the ipad2 users... I confirm that the replying option works with google chrome on my Ipad pro IOS 9.3.
Today I discovered that the problem is fixed for IOS 10 Safari users. I could reply to a thread with my iPad 2017 IOS 10.3.1 I will try later with my iPad Pro IOS 9.3