I am using the slack alert webhook method described here :

https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/media/slack

Everything seems to work except some randoms errors when trying to send slack recover status message. I suspect that when sending multiple and simultaneous message to slack bot, there is some rate limiting on their side, because the problem doesn't appears when there is only one trigger at a second.

Below some screenshots with the errors. The error indicate "channel_not_found" but i do not think this is the real result because as you can see, the other slack alert to the same channel are ok (in green). 

If you have an idea