Ok, I now found the problem... I had to add customers_groups.php to the admin_files-table...
INSERT INTO `admin_files` ( `admin_files_id` , `admin_files_name` , `admin_files_is_boxes` , `admin_files_to_boxes` , `admin_groups_id` )
VALUES ('', 'customers_groups.php', '1', '0', '1');
I don't understand why this wasn't in the i sppc_v41_install.sql, doesn't everyone have to do this? Or is admin_files a osMax-only table?