Steps to reproduce:

1. Generate a new App in Slack and get OAuth token starting with xoxb-*****
2. Add the App to your company in slack, and then add the app to some channel. 
3. Follow media types in Zabbix and set OAuth token to the slack integration
4. Test media type. 

Result:

Error in UI: Slack notification failed : invalid_auth (see screenshot). But getting a message in slack.

Also, I'm getting 3 messages from a real trigger in Slack. I guess Zabbix tries to send 3 times because it gets an error and try to send again and again.

Expected:
Success test and one message in slack about problem.