With the feature "New Source Browser experience" enabled, Bitbucket Notebook Viewer is not able to render correctly. It gives an error "Unable to render notebook" and there is an Internal Server Error on the console. Screenshot as below.

Workaround

• Click Avatar at bottom left corner of the Bitbucket Page
• Click "Bitbucket Labs"
• On the feature "New Source Browser experience", click the slider to disable it
• Notebook Viewer should be able to render properly.