No external diff launches when FileMerge is the selected tool

[Conrad Yoder]

(OS X 10.10.5, SourceTree 2.1, Xcode 7.2)

I have FileMerge set as both my Visual Diff Tool and Merge Tool in ST prefs, but when I select "External Diff" in the contextual menu for a file, nothing happens.

I have installed the Xcode command-line tools using 'xcode-select --install' and I can successfully use opendiff on the command line. (opendiff is in my path as well.) If I select "Install Command Line Tools' from within ST, I get an error dialog:

Installation Failed

Unable to install command line tools, please see the system log for more details.

In the system log, I get the error:

12/16/15 11:29:13.010 AM smd[555]: App "com.torusknot.SourceTreeNotMAS" did not pass helper check "com.atlassian.SourceTreePrivilegedHelper2".

 Any suggestions as to what is wrong here?

EDIT 4/6/16: Still the same problems in OS X 10.11.4, ST 2.2.4, Xcode 7.3.

[hvisage]

Same problem here