SourceTree - UnityYaml doesn't open

I'm working on a Unity 5.3.4f project on a Mac with Git as the version control system and I'm trying to use UnityYaml with SourceTree, so I can deal with conflicts within binary resources (such as scenes and prefabs) with relative ease.

I've followed the instructions on how to set everything up as described in the Unity - Manual: Smart Merge:

1- Added the following piece to my .gitconfig file

 [merge]
 tool = unityyamlmerge

 [mergetool "unityyamlmerge"]
 trustExitCode = false
 cmd = '/Applications/Unity/Unity.app/Contents/Tools/UnityYAMLMerge' merge -p "$BASE" "$REMOTE" "$LOCAL" "$MERGED"

2- Changed the Merge options at SourceTree -> Preferences -> Diff -> Merge Tool / Merge Command

Additionally, I have also changed the Version Control Mode (at Edit -> Project Settings -> Editor) to "Visible Meta Files"

The problem then is: when I try to launch the external merge tool from within SourceTree to solve a conflict, the following message is shown for less than a second:

What is happening? As far as I understood, it was supposed to open UnityYaml so I could solve the conflict, isn't it? Or have I got it wrong?