After installing Mantis, it turned out that the "assigned to", "reported by" and "category" dropdown lists where always empty in the "view bugs" screen. I had to manually add a couple of outer joins in the SQL of file ... (see attached new version).

Functions in "print_api.php" :
-print_reporter_option_list
-print_project_user_list_option_list
-print_project_user_list_option_list2
-print_project_user_list
-print_assign_to_option_list
(possibly not complete, to be checked by the original author of the code)