Can't cancel backups through web interface

Cancelling backups in the stash UI doesn't work. To reproduce:

1 Using the backup client (v.1.3.1), start a backup
1 Go to the UI, and get the maintenance page (http://localhost:7990/stash/unavailable)
1 Provide the token from the logs
1 Click 'cancel backup', and accept the confirmation prompt

Expected results: Backup is cancelled

Actual results: Tomcat error message "HTTP Status 405 - HTTP method POST is not supported by this URL"