Create a new instance of moodle from scratch (maybe an old one is fine too).
Set "Debug messages" to highest level (DEVELOPER...)
Create a new course.
Leave it empty so that only the default forum is in the course.
Go to server > Scheduled tasks
Go to the "Forum mailings and maintenance jobs" and push the link "Run now" in its row.
Confirm the action with the "Run now" button and...

Execute scheduled task: Mail dai forum e operazioni di manutenzione (mod_forum\task\cron_task)
... started 10:04:17. Current memory use 9.3MB.
... used 3 dbqueries
... used 0.0054278373718262 seconds
Scheduled task failed: Mail dai forum e operazioni di manutenzione (mod_forum\task\cron_task),E' stato rilevato un errore di programmazione, deve essere sistemato da un programmatore: Function cron_setup_user() cannot be used in normal requests!
Backtrace:

• line 918 of /mod/forum/lib.php: call to cron_setup_user()
• line 45 of /mod/forum/classes/task/cron_task.php: call to forum_cron()
• line 104 of /lib/cronlib.php: call to mod_forum\task\cron_task->execute()
• line 292 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()

(I apologise for the Italian text but I have no way to change it)