I tried to follow these instruction but was unable, did find another way to fix my problem though.
I too had one forum out of a dozen that wouldn't allow me to post using my iPad. I could log in, surf, even reply to PM's using my iPad, but when I tried to reply to topics in a thread, the keypad wouldn't open when I clicked in the "reply" window, and when I clicked on the "instant reply" window the browser would shut down.
I found that by logging into the forum, I could then click on my username to bring up my forum options - My Profile, My Settings, My Content, Messenger, Manage Friends, etc.
Clicked on "My Settings"
This brought up Time Zone, Posting Settings, and Search Settings.
I noticed in "Posting Settings" there was a checkbox for "Enable visual (RTE) editor?" and this check box was ticked. I unchecked it (thus DISABLING visual RTE editor) and it solved my problems.