imagecache-htaccess problems

Hello all, i'm using the imagecache module and its working great but i'm having a little problem with it..

i'm also using the phpbb module and cause of that i have my file system folder like this:
forum/shared/imagecache/

now the phpbb module comes with a htaccess file:

#drupal rewrite rules
<IfModule mod_rewrite.c>
  RewriteEngine on
  # Modify the RewriteBase if you are using Drupal in a subdirectory and
  # the rewrite rules are not working properly.
  #RewriteBase /
  
  #RewriteCond %{REQUEST_FILENAME} !-f
  RewriteRule ^inbox$           ucp.php?i=pm&folder=inbox [L]
  
  #RewriteCond %{QUERY_STRING} ^mode=login(.*)$
  RewriteCond %{QUERY_STRING} !^bypass=true(.*)$
  #RewriteRule ^ucp.php(.*)$ ../phpbb/login [L]
  
  RewriteCond %{QUERY_STRING} ^mode=register(.*)$
  RewriteRule ^ucp.php(.*)$ ../user/register [L]

  RewriteCond %{QUERY_STRING} ^mode=logout(.*)$
  RewriteRule ^ucp.php(.*)$ ../phpbb/logout [L]
  
  RewriteCond %{QUERY_STRING} ^mode=sendpassword(.*)$
  RewriteCond %{QUERY_STRING} ^mode=resend_act(.*)$ [OR]
  RewriteRule ^ucp.php(.*)$ ../user/password [L]
</IfModule>

now when the htaccess is enabled the imagecache wont get acces to the thumbs and wont show them ....

i hope sombody can help me