Steps to reproduce:

1. Configure a Media Type; email or slack. In this case slack was used as an example. Install the zabbix-notify script from the following github project on your Zabbix server - https://github.com/v-zhuravlev/zabbix-notify.
2. Set up a Zabbix Media Type with the required script parameters.
3. Set up a Slack webhook to send the data into slack from Zabbix.
4. When a trigger comes in Acknowledge it.
5. You will see "Inaccessible user" for the name while the Zabbix UI (Monitoring > Problems) will show who acknowledged it.

Result:
See screenshots. This was also seen in email and not just slack.

Expected: When a user acknowledges the trigger, the media configured should display the user who acknowledged it and not just the UI.

Setup:

• Zabbix 4.0.15
• Debian 9 OS
• PostgreSQL 11.4

Could this be a possible permissions issue?