Navigate to C:\Users\UserName\AppData\Local\Atlassian

Under there, you will see SourceTree.exe_Url_RandomGuid

Optional: Take a backup of the user config file in SourceTree.exe_Url_RandomGuid

Delete the folder SourceTree.exe_Url_RandomGuid so you are only left with one folder called SourceTree.

Start SourceTree, and it will rebuild that settings file.

Optional: Close SourceTree, restore the user config file. Open SourceTree  

Go into the C:\Users\UserName\AppData\Local\SourceTree folder and you might see two folders: app-3.4.9 and app-3.4.10. One of the two should be empty. Delete it, and SourceTree should work again.

If this doesn't work, check the C:\Users\UserName\AppData\Local\Atlassian folder. 

Once there, delete the folder SourceTree.exe_Url_RandomGuid so you are only left with one folder called SourceTree. 

Start SourceTree, and it will basically reinstall Sourcetree.