Did you yourself try to reply to that thread and get the same message? Notice the triple post by Tagerith, he probably had the same debug message and hit the submit button 2 more times thinking the first submission didn't go through.
I did, yes - it enters the post in the database, but for some reason when displaying the thread it didn't pick up the new posts and display them in the forum; for some reason the new ones could be accessed from the 'latest post' on the forum index and were displayed in 'Page 4 of 3'!!
I suspect what may have happened is that someone in there may have have been using the spanish template on the old forum (1.4.4) and it wasn't updated correctly - in this version of the firum there are currently only English templates.