SourceTree asks for Bitbucket login every few minutes

I use Bitbucket to connect to Git and Mercurial repositories on Bitbucket and GitHub.

When I leave SourceTree open, once every five minutes it displays a "Bitbucket Login" dialog asking for my username and password for Bitbucket. If I close the dialog without entering my username and password, the dialog immediately reappears. The only way to get the dialog to go away is to enter my username and password, but even then, it will come back in 5 minutes.

This bug severely disrupts my work, since the Bitbucket Login dialog interrupts what I am doing every time it pops up. SourceTree should not need me to log in once every 5 minutes. The login dialog should never immediately reappear if I close it.