At first, please check the Java Console of your browser. If you have an error message similar to this

Can't open lex/userdic.tlx(url,t)
Can't open lex/sscefi.tlx(url,t)
Can't open lex/sscefi2.clx(url,c)

the problem can be solved as follows:Windows 2003 Web Edition allows only listed MIME types by default. If you create a new web site with default settings, you can not use rare file types without adding them first in IIS console. After adding .tlx and .cls from “Sites Properties” – “HTTP Headers” – “Mime Types”  the spelling check should work correctly.