I haven't (so far) had any problems replying to posts, but I have noticed over the last few days that some bits of the forums are rendering oddly. Some examples:
Threads that run over one page now have the number page links listed vertically, not horizontally, so such threads get a longer block (with lots of empty space) in the forum listing.
In each forum post, the member's icon is either slightly larger than before, or slightly shifted down, so it now covers the top half of the first line of the member description, e.g. "AD Author".
The pin icon against the title of pinned threads seems to be jammed up against the text in a way that it wasn't before.
In the main forum listing, the text indicating the date of the most recent post in each forum has a stray space + comma (i.e. " ,") on the end.
My guess is that the CSS or equivalent has been subtly altered in some way - a recent Invision update? If it helps for diagnosis, my browser is Firefox v. 80.0.1, running on Windows 10 v. 2004.