Hi Malte, thanks for your comments.
Our idea is to add user_id and community_id to each request stored in the logfiles. Awstats will use REGEXP to filter the URL and then to recognize objects visited, e.g. when a user wants to see a message of a forum, it can be identified by message-view?message_id=XXXXXX.
As first approach, we are going to use the data provided by awstats. This software, extracts the important information of logfiles, and stores it in formatted text or xml files.