Internal Server Error - every 2 hours

This is the error showing in frappe.log:

New Exception collected with id: 2017-01-24 10:00:23.867631-117.198.96.5-d16
[ERROR] 2017-01-24 10:00:25,351 | /home/frappe/frappe-bench/apps/frappe/frappe/utils/scheduler.py:
Site: site1.local
Exception in Enqueue Events for Site site1.local
Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/frappe/frappe/utils/scheduler.py”, line 64, in enqueue_events_for_site
if is_scheduler_disabled():
File “/home/frappe/frappe-bench/apps/frappe/frappe/utils/scheduler.py”, line 214, in is_scheduler_disabled
return not frappe.utils.cint(frappe.db.get_single_value(“System Settings”, “enable_scheduler”))
File “/home/frappe/frappe-bench/apps/frappe/frappe/database.py”, line 550, in get_single_value
tabSingles where doctype=%s and field=%s"“”, (doctype, fieldname))
File “/home/frappe/frappe-bench/apps/frappe/frappe/database.py”, line 106, in sql
self.connect()
File “/home/frappe/frappe-bench/apps/frappe/frappe/database.py”, line 52, in connect
use_unicode=True, charset=‘utf8’)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/MySQLdb/init.py”, line 81, in Connect
return Connection(*args, **kwargs)

File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/MySQLdb/connections.py”, line 193, in init
super(Connection, self).init(*args, **kwargs2)
OperationalError: (2002, ‘Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111 “Connection refused”)’)