Steps to reproduce:

**Default settings media type "Slack", set bot_token and zabbix url.

Result:

When there is a Trigger Event (some problem on monitoring servers), in event log of Zabbix get an error "Slack notification failed : invalid_auth" and uses all 3 tries, but all notifications apears in slack.
When problem resolved, in event log of Zabbix get an error "Slack notification failed : channel_not_found", messages don`t update in slack.

See log file zabbix.log