Steps to Reproduce:
- Activate "mySandbox: Add a “Sandbox” link to the personal toolbar area." on Wikidata.
- Have your /sandbox page created.
- Click "Sandbox" in the personal toolbar area.
- You will see a blank page (status "200 OK").
- You are redirected to your /sandbox page.
If the sandbox does not exist, it works as expected. The cause is combination of Special:MyPage and action=edit&redlink=1, these two components are sufficient for reproducing.