To reproduce....

1. in Server > Mail, set the server to something invalid (to force a connection error)
2. in a course, ensure some users are enrolled, create a forum and force subscribe users
3. Do a test posting and check "Mail now"
4. On the command line run 'php /path/to/moodle/admin/cli/cron.php'

Result is...

ERROR: SMTP Error: Could not connect to SMTP host.
++ Error: Could not insert a new entry to the Moodle log ++

• line 1695 of /lib/datalib.php: call to debugging()
• line 4741 of /lib/moodlelib.php: call to add_to_log()
• line 63 of /message/output/email/message_output_email.php: call to email_to_user()
• line 152 of /lib/messagelib.php: call to message_output_email->send_message()
• line 644 of /mod/forum/lib.php: call to message_send()
• line 87 of /lib/cronlib.php: call to forum_cron()
• line 61 of /admin/cli/cron.php: call to cron_run()
  ++ Error calling message processor email ++
• line 153 of /lib/messagelib.php: call to debugging()
• line 644 of /mod/forum/lib.php: call to message_send()
• line 87 of /lib/cronlib.php: call to forum_cron()
• line 61 of /admin/cli/cron.php: call to cron_run()

An intermittent failure to access an SMTP server is fairly common so we have already seen this logging error a few times.