Have a look at the following thread:
Problems with Forum when using <>
The problem you are facing is due to the fact that mod_security is disabled. This is because injection/hacking attacks can be launched using insecure code + HTML. From what you say, I take it that this is a PHPBB forum, if so I highly suggest that you update your forum to PHPBB 3; that should iron out any problems. If you don't want to update your forum and need the HTML editing, you can open a ticket with support and ask to be moved to a server (firefly) which is available for customers who need certain disabled PHP functions among others.
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Regards,
Josh Hold
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