Bitmask Network Error

When I launch the Bitmask GUI, I see a popup with the following Network Error message:

Error: Network Error

at XMLHttpRequest.req.onerror (http://localhost:7070/app.bundle.js:31342:25)

I can close the error message, but the main bitmask window remains completely dark, showing no control elements at all.

The following message is printed, when started from command line:

[+] launching bitmaskd...

[+] registering python<->js bridge

js: Warning: Accessing PropTypes via the main React package is deprecated, and will be removed in React v16.0. Use the latest available v15.* prop-types package from npm instead. For info on usage, compatibility, migration and more, see https://fb.me/prop-types-docs

js: Warning: Accessing createClass via the main React package is deprecated, and will be removed in React v16.0. Use a plain JavaScript class instead. If you're not yet ready to migrate, create-react-class v15.* is available on npm as a temporary, drop-in replacement. For more info see https://fb.me/react-create-class

js: Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of ErrorMessage. See https://fb.me/react-warning-keys for more information.

in li (created by ErrorMessage)

in ErrorMessage (created by ErrorPanel)

in div (created by ModalBody)

in ModalBody (created by ErrorPanel)

in div (created by ModalDialog)

in div (created by ModalDialog)

in div (created by ModalDialog)

in ModalDialog (created by Modal)

in Transition (created by Fade)

in Fade (created by Modal)

in div (created by Modal)

js: Warning: Modal: isMounted is deprecated. Instead, make sure to clean up subscriptions and pending requests in componentWillUnmount to prevent memory leaks.