Play/pause buttons launches iTunes instead of playing/pausing the Deezer app
Found the answer on the Deezer community forum:

You have to allow Deezer the accessibility rights, go to:


System Preferences
Security and Privacy
Privacy tab
Choose "Accessibility"
Add "Deezer" to the allowed software


You'll have to unlock the configuration by clicking on the lock and enter your system password.

The original answer can be found here.