As of SourceTree 1.6.3, when I switch between branches across which file system file and/or folder renames have been performed (e.g. /src/Car/* has been moved to /src/Vehicle/*) or even a simple set of renames within the same folder (e.g. all /src/Car/ files get renamed from Car<name> to Vehicle<name>) I get a series of:
git -c diff.mnemonicprefix=false -c core.quotepath=false checkout <branchname>
error: cannot stat 'src/Car': Permission denied
errors. In addition the location itself becomes inaccessible via the filesystem. The /src/Car folder is unreadable, showing a "You must have Read permissions to view the properties of thos object. Click Advanced to continue." All subsequent operations in File Explorer result in the same error. No amount of fiddling seems to unlock the folder in question.
A reboot resolves the matter. Switching between branches immediately works as expected.