You will find a similar behavior with old versions of GitHub for Enterprise, as illustrated in the issue 50.
The recommendation, for forks on GitHub, is to wait a few hours after deleting the previous fork, and try again (clicking on that fork button).
Note that you shouldn't have to delete a fork if you intent to make new pull requests: simply isolate them in their own branch after having updated your fork (pushing to origin the latest from upstream).
See also (to avoid deleting a fork):