Hello,
I think I have it now. After two days looking into coppermine's code and understanding its user management, I changed the bridge to work with it (not using user levels or something, changed all to use phpbb3 groups).
Now you can use the post-based groups (set "post_based_groups" in the config table of the bridge to "1") and configure each of them.
This means you aren't bound to the standard groups of coppermine and can define restrictions for every group as defined in phpbb3 (RC3).
If you don't need this, leave the value for "post_based_groups" at "0".
Please note that switching between using post-based groups (or not) causes the loss of your group configuration data (max. upload space etc.), so you have to go to the group manager (link "groups") and update the group settings after switching. (This isn't a big piece of work)
If you want to enable a group defined in phpbb3 to have admin rights, add its group id to the initialisation of the admingroups array in constructor:
E.g. if you want to give global moderators (normally having ID 4) admin rights, change the line:
$this->admingroups = array($BRIDGE['admin_group']);
to
$this->admingroups = array($BRIDGE['admin_group'], 4);
I attached the reworked bridge file, please not that I tested it with the RC3 of phpbb3 only.
@gele: you're right - the board url must end with a slash, otherwise redirection won't function
@gassnerj: I wonder about changing the name of class instance, I ever thought this doesn't depend on Coppermines' table prefix.
I don't think your cookie settings aren't right, I think you haven't placed the correct cookie prefix into your bridge configuration table. This cookie prefix is different to every phpbb3 installation, so you can't use my posted one and have to take the value from phpbb3's config table 'cookie_name'.