can not use phpbb integration module when phpbb and drupal are installed on different subdomains

Hi.

I have a working installation of phpbb on a subdomain. (drupal.ehsvenlo.nl)
I have a working installation of drupal on a subdomain. (phpbb.ehsvenlo.nl)

I want to combine them, so I started testing.
I created exactly the same setup on another domain I own. This other domain is exactly the same as the one I am using.
I newly installed phpbb and drupal. Both with admin as administrator account, both with the same password and email address.
I have altered any settings. Both sites are completely standard. No settings altered not during install, neither post installing.
I install the module, as described in the txt file. And everytime (yes i have tried several times) I get the "anonymous" authentication error (http://drupal.org/node/225566). No matter what I try, I can not get it work.

Then as a final resolve, I tried installing drupal and phpbb on the main domain. (both in a separate directory (phpbb and drupal))
Again, I installed completely standard, no settings altered during or post installation.
I installed the module as described in txt file. And immediately it worked as it should. No error.

What do I have to do to get it to work when phpbb and drupal are installed on different subdomains?

The versions I use are:
phpbb 3.02
drupal 6.3
phpbbforumintegration 6.x1.03

Please help

best regard,
Twan Veugelers