Resolving conflicts with external merge tool using Araxis Merge broke for me in the past two weeks. Araxis Merge does not start. Perhaps I upgraded Araxis Merge recently - I can not remember. I am using Araxis Merge 2018 Pro (2018.4971) for "El Capitan or later" and SourceTree 2.7.2 (167). I am using High Sierra - that may be the culprit.