Well, I'm guessing that the site is implemented using several controls. The control used to accept your reply to a post is different from the one accepting updates to your signature. So we have managed to identify that there is a problem with the former which is not expressed in the latter.
What I have more difficulty explaining is why it works for me and not for you (and several others). It may not be simply co-incidence that the forum members who cannot create links are also having a problem with their avatars. Moreover, these individuals seem to be newer members. This would seem to indicate there are indeed some user settings associated with a user and perhaps that the default values for these settings have changed.
Unfortunately without access to the code, there's not much more that I can say or do. What I would say is that the instructions in my format posting thread are correct and represent the simplest of html. However, the html encoding being performed by the site are preventing the required result from being displayed.