Ok, I solved it myself.
It is not working with PHPBB right away.
1. Variable $data is used in BotScout but also by phpbb.
2. If register.php is opened to show the form, it already queries the BotScout API. I added an if-clause, which does not start the check, if $XMAIL is empty before sending the query.
3. in // sanity check the API return is printed on the screen, but this conflcits with the later /includes/functions.php try to add something to the header. Since this output is not needed, I also added a if diag=1 in here.
Now it is working!
Thanks for developing this tool!
Maybe you like to add those changes to help other phpbb users.