Since upgrading to Sourcetree 3.3.4, we are getting an error message when trying to create a pull request.
Also, the "Remote" button, located on the top right of the Sourcetree window, is now showing a red exclamation mark. When this button is clicked another error message is shown "Unable to open your web browser to <remote URL>, do you have a default web browser set?"
Going to Setting as suggested by the first error above, selecting the your remote, clicking "Edit", then without changing anything, clicking OK to close that window, and then OK again to close the "repository settings" window, fixes the error and pull requests can be created again, but only temporarily, until Sourcetree is closed.
The next session of Sourcetree will still show the error when creating a pull request.
Note: This error happens in version 3.3.4, not in version 3.2.4, but when creating the issue here and typing 3.3.4 in the "Affects Version/s" edit box, I get an error saying that that version doesn't exist.
There's a community question related to this: