Issue: OctoSlack Plugin throws SSL verify error in case of using self-signed server certificate for the Mattermost instance. Solution: Edit /home/pi/oprint/bin/octoprint and add the highlighted lines to make Python use servers default CA bundle file (must include the self-signed root CA then):

import sys import os from pkg_resources import load_entry_point

os.environ['REQUESTS_CA_BUNDLE'] = '/etc/ssl/certs/ca-certificates.crt'

if __name__ == '__main__':