problems with phpbb module and imagecache module

Windows XP
apatche 2.2.4
msql 5.0
php 5.2.3
drupal 5.2
phpbb module for phpbb3
imagecache 1.3

I have a problem. And I am working on it now, for 4 days to get the imagecache working on my site. I did everything I found on the drupal site to get it working.
Today I found that with the integration of phpbb I changed the .htaccess file of the forum, to

RewriteEngine on
# Modify the RewriteBase if you are using Drupal in a subdirectory and
# the rewrite rules are not working properly.
#RewriteBase /

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]

As written in the phpbb module install description. I remove this and it works fine.
The problem here is that the shared directory is placed in the forum directory so that it can share avatars and files with the forum. The directory is :
'forum/shares'

I tried some stuff today but everything makes it worse. The thing is that when the thumbnails are there it works. But the creation of the thumbnails by imagecache fails. It seems that every Rewriterule that is done in the forum .htaccess file stops imagecache from creating thumbnails.

Anybody any idea how to fix this ?

AmigoB