Its common practice to provide a demo login so that people here don't have to register on your board in order to help you. e.g.
Demologin Username: demo Password: test
Have you checked whether or not you receive a reply notification for posts you are subscribed to on your board. The problem may be related to the way email is setup.
I don't have an international version installed anywhere so I'm not 100% familiar with that version but you could try this.
In post_info.asp AFTER the includes (e.g. <!--#INCLUDE FILE="inc_func_count.asp" --> ) near the beginning of the page include this line;
on error goto 0
Then try to post a reply. It should force the webserver to return a specific error and probably prevent users from posting replies. You will want to edit out that line immediately after you get the error message in order to allow users to post again.
There is probably still an error happening. If you want to find out what it is you would have to replace all the "on error resume next" lines to "on error goto 0" until you force the error to be displayed.
Its up to you. If you're happy with it working then don't break it again.