Go to http://www.bdsm.com.tw/forum/search.php?keywords=%E5%8F%B0%E4%B8%AD

It says

The following words in your search query were ignored because they are
too common words: 台 中. You must specify at least one word to search
for. Each word must consist of at least 3 characters and must not
contain more than 14 characters excluding wildcards.

That is not true.

The default configuration of phpbb is not ready for unicode!

I am reporting a bug about the default configuration of phpbb.

Do not ask me to contact the administrator or change the site.

The problem is in the default configuration of phpbb.