I'm getting "Invalid redirect_uri" error during the registration using my Bitbucket account.
It seems like it goes through if I manually replace the characters in redirect_uri but the installation window goes back to the Registration step and can't go to the next step.
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.
It looks like you all might be running into this:
While there isn't a posted solution for it yet, you might want to go vote, watch and comment on that issue.
Sorry I can't offer more help.