Replies not working properly

I have the exact same problem as this post:

https://forumwpplugin.com/topic/post-and-reply-module-malfunctioning-after-update/

A black box with a red line appears on reply submit. The reply does not appear until some time later and then appears multiple times as the user has hit submit several times to see their reply.

Please advise how to fix.

This is a new install BTW with Ultimate Member theme being used.