I would ensure you are running Git 2.20.1 or higher, either but updating Sourcetree to v2.0.17 and then updating the embedded Git in the Tools/Options/Git tab or by installing Git for Windows 2.20.1 directly and configuring Sourcetree to use it as a System Git in Tools/Options/Git.
There were some problems with the Git Credential Manager GCM that is bundled with Git for Windows and that error sounds like one of them
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