I've been using Bitbucket in conjunction with SourceTree on this machine for several months with no problems. After updating the OS to Mojave, the "Checkout in Sourcetree" button would only open Sourcetree but would neither fetch nor checkout the branch. More recently, in the last couple of days or so, it won't even open Sourcetree. Now today, fetching in Sourcetree does not retrieve branches created directly in Bitbucket. It's as though they don't exist.
I don't know if Atlassian folks are aware but everybody look to be impacted. 3.0.1 is buggy too.
As a workaround, downgrade to version 2.*