Since deleting the local branch "BranchName" by right+clicking -> Delete BranchName, I get the above error message on every command I trigger from SourceTree. Everything seems to work though.
The branch did never exist on the remote and is deleted locally. Using git from the command line is working without complains.
git branch does not list the deleted branch. I "git clean -fd". "git log" also works with no issues.
System:
OSX 10.9.2
Sourcetree Version 1.9.4 (1.9.4) (update installed today)