This is similar to [1] but bear with me, please.
I'm running 2.1.0 (2cd006474ec8a808).
Tried the slack-notifier gem and it was throwing 404 (I had taken the URL from Mattermost and it looked correct.
Tried using cURL with curl -i -X POST -d 'payload={"text": "Hello, this is some text."}' http://mydomain:myport/hooks/123456123456abcdef (taken from [1]) and it also threw 404's.
Then I saw the reason in the logs:
[2016/03/22 23:52:32 UTC] [EROR] /hooks/deadbeef: code=404 ip=1.2.3.4
[2016/03/22 23:52:32 UTC] [EROR] Error rendering template error err=html/template:error.html:4:11: no such template "head"
[2016/03/22 23:52:32 UTC] [EROR] Failed to create error page: html/template:error.html:4:11: no such template "head", Original error: Handle404: ,
It seems it's trying to render an error page, so it might be masking another error behind it...
Will keep looking.
-- nachokb
[1] http://forum.mattermost.org/t/mattermost-log-error-404-with-integration-hooks/479