[PinkMilk]

If its just the one post its likely the post content, Edit the post, copy/back up the content somewhere (stick it in a txt file) then post some random text and save.

Heres some random text

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

Now test the reply quote button, if it now works its the post content messing with the background code somehow (possible XSS vulnerability orr something like that ) and you will need to work which part of the post is doing so and repost with edits made.