This may be more related to Discourse, rather than these particular forums. But I encounter it often.
When replying to a post (or creating a topic), the editor simply doesn’t open.
A way around that is to close the tab and open it again. Sometimes it works on the first try, sometimes the operation has to be done 5, 6 or more times. (I’ve found that “open in new tab” works best for when viewing topics)
There is another forum I also frequent that uses Discourse and I experience the same bug there. I’ve tried this with multiple browsers such as Firefox, Chrome and Edge.
Edit: Now solved. The problem was entirely in the PEBCAK-protocol.