Just to say that we are seeing this problem in the Fairphone forum too, seemingly since updating to v.2.9.0 beta 5 and 6. You’ll find a thread here.
I confirm this seems only to happen when the new post is begun by clicking the grey “Reply” button of a preceding post. Using the blue general Topic Reply, or using “Quote” to initiate the post, does not produce the issue.
We observe other (JS - related?) issues at the same time, for example the “Quote”, and inserting a hyperlink (the optional title field is not populated and the dialogue cannot be validated). These issues are not seen if the post was not initiated by clicking a grey “Reply” button.
I also confirm that the issues have not been seen in safe mode.
That means that there are error in the Fairphone theme. It must be fixed by an admin there. It appears that, for example, there is a theme-component trying to use the old deprecated Discourse.User global object.