SourceTree is using IE in the background to get to https://id.atlassian.com, so you need to figure out why IE is blocking that site. Make sure you use a supported version of IE.
Checkout this thread and see if it helps, https://community.atlassian.com/t5/Sourcetree-questions/Can-t-login-during-Install-Windows-7-on-corp...
There is also an issue logged for it, SRCTREEWIN-7949, check it out and also add yourself as a watcher in case there is a workaround or a fix for it.
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