Hi
This is due to a bug in the Git Credential Manager for Windows which ships with Git for Windows.
Quickest way to fix is to install Git for Windows 2.19.1 and then install Git Credential Manager 1.18.3 https://github.com/Microsoft/Git-Credential-Manager-for-Windows/releases/tag/1.18.3 then set Sourcetree to use this install in the Tools/Options/Git tab System Git option,.
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