After migrating to a new server I get an error in the Forum cron job (see below). What's causing it?

Execute scheduled task: Forum mailings and maintenance jobs (mod_forum\task\cron_task)
... started 20:21:50. Current memory use 22.2MB.
Coding problem: $PAGE->context was not set. You may have forgotten to call require_login() or $PAGE->set_context(). The page may not display correctly as a result
line 493 of /lib/pagelib.php: call to debugging()
line 1544 of /lib/pagelib.php: call to moodle_page->magic_get_context()
line 677 of /lib/pagelib.php: call to moodle_page->initialise_theme_and_output()
line 864 of /lib/pagelib.php: call to moodle_page->magic_get_theme()
line 466 of /mod/forum/lib.php: call to moodle_page->get_renderer()
line 45 of /mod/forum/classes/task/cron_task.php: call to forum_cron()
line 105 of /lib/cronlib.php: call to mod_forum\task\cron_task->execute()
line 297 of /lib/cronlib.php: call to cron_run_inner_scheduled_task()
line 91 of /admin/tool/task/schedule_task.php: call to cron_run_single_task()
... used 5 dbqueries
... used 0.028049945831299 seconds
Scheduled task failed: Forum mailings and maintenance jobs (mod_forum\task\cron_task),Coding error detected, it must be fixed by a programmer: Function cron_setup_user() cannot be used in normal requests!