Adding and event to a calendar cases "Coding error detected, it must be fixed by a programmer: Invalid context level of parameter."

Only on the home page adding an event to "ALL" works fine, but selecting a course causes this error. I turned on Deleveloper Level debugging messages and get this.

Coding error detected, it must be fixed by a programmer: Invalid context level of parameter.

Stack trace:
•line 3956 of /lib/accesslib.php: coding_exception thrown
•line 3089 of /lib/accesslib.php: call to get_course_context()
•line 77 of /blocks/online_users/block_online_users.php: call to get_enrolled_sql()
•line 279 of /blocks/moodleblock.class.php: call to block_online_users->get_content()
•line 232 of /blocks/moodleblock.class.php: call to block_base->formatted_contents()
•line 882 of /lib/blocklib.php: call to block_base->get_content_for_output()
•line 934 of /lib/blocklib.php: call to block_manager->create_block_contents()
•line 342 of /lib/blocklib.php: call to block_manager->ensure_content_created()
•line 7 of /theme/splash/layout/general.php: call to block_manager->region_has_content()
•line 627 of /lib/outputrenderers.php: call to include()
•line 585 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
•line 215 of /calendar/event.php: call to core_renderer->header()

as admin or a teacher, this error happens. When I switch templates back to stadard, same error occurs.
My moodle is a fantastico installation.
I have found a forum entry talking about the same problem.
http://moodle.org/mod/forum/discuss.php?d=165451#p736608 (in french)