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This is a known issue and Atlassian is working on it, see this thread for more info, https://community.atlassian.com/t5/Sourcetree-questions/Cannot-login-to-bitbucket-in-order-to-use-sourctree/qaq-p/1100226.
Sorry to hear about this problem with logging into a Bitbucket account from Sourcetree. There is currently an outage in regards to Bitbucket Cloud via OAuth logins are not working correctly. This is the cause of the inability to login here and in turn showing that 'Invalid redirect_uri' error message.
Atlassian has just updated our StatusPage about this incident that you can use to track this problem in https://bitbucket.status.atlassian.com/incidents/l50d1j43zdj9
I would recommend subscribing or watching that incident for update. Once this incident is resolved I would expect that you should then be able to get Sourcetree to login to Bitbucket again via this OAuth method.
Sorry again for the inconvenience here.
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