I am using SourceTree 2.7.1 on a Mac with OS 10.13.3 connecting to TFS 2015 Update 3 with the Git service/Plugin.
I repeatedly get challenged for my password. Sometime I get challenged multiple times in a row. The password works, but if I am challenged more that x times, I am sometimes locked out of my account.

I have cleared out the "Default usernames for URLS which do not include one:" on the Advanced tab in preferences.