error on some posts

[iran.gs]

why every once in a while i get this when post something as admin or as member

every once in a while i get this error
Forbidden

You don't have permission to access /forum/newthread.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

no here is a example of the post i was going to post in my site let see if same error come for u

this happens on big lists or sometimes on even small ones some time i rename some stuff or cut some words or $%#^#^ or etc it will be ok after that but still sometime even on real small post this happens

here is an example

forest_grumps_8de5fbf3__fp_.avi
http://rapidshare.de/files/33331785/pokemon_johto_journeys_-_158_-_the_psychic_sidekicks_3158e96c__fp_.avi
http://rapidshare.de/files/33333121/pokemon_johto_journeys_-_159_-_the_fortune_hunters_ea7a144d__fp_.avi
http://rapidshare.de/files/33284248/Pokemon_Johto_League_Champions_-_160_-_A_Goldenrod_Opportunity_pe__BF7BC33B_.rar
http://rapidshare.de/files/33284923/Pokemon_Johto_League_Champions_-_161_-_A_Dairy_Tale_Ending_pe__FBC2F9DD_.rar
this will give me an error