A similar issue is described here however, my symptoms are a little different and none of the steps there seemed to work. Nor does the suggestion here work.
I\'m attemp
I had same problem. My fix way:
Remove user from SourceTree settings (optional, i not sure);
Add you account in setting and generate new SSH key (it's a main part of fix);
Clone your repo;
If SourceTree sill can't clone repo you can try to clone from main page of SourceTree choosing external repos.