With an existing repo (well over a year using SourceTree with it) SourceTree loses the ability to create a pull request on GitHub.
When right-clicking a branch and choosing "Create pull request", I'm presented with this dialog
The only way to get it working again is to hit the Settings button (or go into Repo settings), edit the GitHub entry, and click OK to save. You don't have to make any changes to the settings, just edit origin and click OK.
This is a private GitHub repo, using SSH. Extended integration is properly set to GitHub and nothing has changed with the remote. Once I go through the motions to fix this, it keeps working for a few weeks and then reoccurs. It happens while SourceTree is running, so it isn't a startup/shutdown issue.