Did you also edit the previously created post as hosseinimr93 explained? The issue only becomes apparent after an edit of a post that contains links where there are empty spaces before the link which are dividable by 2.

It's no secret that theymos has heavily modified the current forum software to make it safer and more usable for what he wanted to achieve. One of those custom changes might have led to this "bug" or it's a known issue that got fixed in later SMF versions like the one you are using on your forum.