With debug set to developer mode, I get the following error when logged in as an administrator and I click on the "By slot duration" tab:
Invalid get_string() identifier: 'appointments' or component 'scheduler'. Perhaps you are missing $string['appointments'] = ''; in mod/scheduler/lang/en/scheduler.php?
line 293 of /lib/classes/string_manager_standard.php: call to debugging()
line 6838 of /lib/moodlelib.php: call to core_string_manager_standard->get_string()
line 212 of /mod/scheduler/viewstatistics.php: call to get_string()
line 80 of /mod/scheduler/view.php: call to include()