I am having the same issue. I have created my bitbucket account using my google account. Now Source Tree requires my password to connect with bitbucket. I only have the option to change my password - which requires my old password - so this isn't possible.
Hi, I was stuck with the same problem and found a stupidly simple solution. Login to your account using google, then If you're using bitbucket then reset you password here or if its an atlassian account like in my case, reset your password here. It won't ask you for the old password. :)