Hi Accord. I honestly don't know why you get that error. I'm sure there are other forums on ForumUp using Thai fonts, without problems.
In my experience from the Vietnamese support forum, I know that different browsers do not only read the fonts differently, but they also write them differently. The Vietnamese Moderator translated stuff, and I copied and posted it, and got errors; but different errors with IE7 and Firefox.
But I guess that a similar problem for you would have been visible in the message bodies, and not only in the sub forum titles.
And it seems to work on the index page, which rules out the link code as the cause of the error.
I'll try to do some research, and I suggest that you do the same, as it most probably is more easy to find something about it in Thai, than in English.
And perhaps someone else in the Staff knows more...
FoxLeonard