crm_mon -E does not run the external-agent on Debian Jessie

Question

Setup

I'm currently having a working 2-nodes HA cluster using Pacemaker+Corosync. My nodes are running on Debian 8 (Jessie). Now I would be able to get notified when changes occur in the cluster (resources stop/start, promote/demote, move...).

Since email reporting is so-2008, I'd like to use Slack. To do that, I created a script that uses curl to post a message to my team's Slack channel, using Slack's webhooks. My script is using environment variables that are documented here: Pacemaker - 7.3. Configuring Notifications via External-Agent.

The script works fine when executing manually in the shell and is able to post on the specified channel. It also logs to /var/log/ocf-notifier.log.

Problem

Based on this answer, I created a new resource in the cluster using Pacemaker's ocf:pacemaker:ClusterMon resource agent, which calls my custom script (/usr/local/bin/ocf-notifier).

However I noticed that my script is simply not called at all when changes occur in the node (tried stopping resource as well as completely shutting down a node).

Therefore, I tried to launch crm_mon by hand like the following:

$ crm_mon -Arf --interval=2 -E /usr/local/bin/ocf-notifier -e '@jordan'

And see if this could trigger the thing by playing around with the cluster with another shell. As it turns out, crm_mon was able to see the changes happening in the cluster (node going offline, resource being stopped/started...), but my custom script never blinked an eye. My custom log file remains empty, and nothing appears in Slack since I believe the script is simply not called.

TL;DR

crm_mon does not call the external agent on cluster events, as it should with -E option. What am I doing wrong?