I added a lot of code to the write method in the project.task model. I need to figure out what's causing the error and for that I need to understand the error. Can someone tell me what this error means or when it's normally raiser.

The whole error message:

Traceback (most recent call last):   File "werkzeug\serving.py", line 177, in run_wsgi    
  File "werkzeug\serving.py", line 165, in execute    
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\service\server.py", line 246, in app
    return self.app(e, s)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\service\wsgi_server.py", line 186, in application
    return application_unproxied(environ, start_response)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\service\wsgi_server.py", line 172, in application_unproxied
    result = handler(environ, start_response)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\http.py", line 1308, in __call__
    return self.dispatch(environ, start_response)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\http.py", line 1282, in __call__
    return self.app(environ, start_wrapped)
  File "werkzeug\wsgi.py", line 579, in __call__    
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\http.py", line 1486, in dispatch
    response = self.get_response(httprequest, result, explicit_session)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\http.py", line 278, in __exit__
    self._cr.commit()
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\sql_db.py", line 154, in wrapper
    return f(self, *args, **kwargs)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\sql_db.py", line 378, in commit
    func()
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\addons\mail\models\res_partner.py", line 248, in send_notifications
    env['mail.mail'].browse(email_ids).send()
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\addons\mail\models\mail_mail.py", line 297, in send
    mail._postprocess_sent_message(mail_sent=mail_sent)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\addons\portal_sale\models\mail_mail.py", line 23, in _postprocess_sent_message
     return super(Mail, self)._postprocess_sent_message(mail_sent=mail_sent)
File "D:\Program Files (x86)\Odoo 10.0\server\odoo\addons\mail\models\mail_mail.py", line 141, in _postprocess_sent_message
'email_status': 'sent',
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\models.py", line 3557, in write
    self._write(old_vals)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\models.py", line 3643, in _write
    cr.execute(query, params + (sub_ids,))
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\sql_db.py", line 154, in wrapper
    return f(self, *args, **kwargs)
  File "D:\Program Files (x86)\Odoo 10.0\server\odoo\sql_db.py", line 231, in execute
    res = self._obj.execute(query, params)
TransactionRollbackError: could not serialize access due to concurrent update