[6.8.3 -> 6.9.0] Docker won't start, vDisk location red

I updated to 6.9.0, and now when I go to start Docker (having stopped it to install the Nvidia plugin, ala what SpaceInvader told me to do), all that happens is the Docker vDisk Location field goes red, despite the contents ending in .img "/mnt/cache/Docker/docker4.img". I haven't changed any of the settings. Any ideas? I'm hoping this is a stupid typo, but right now it is immensely frustrating.

Edit: to add, I have tried in multiple browsers, same issue. Nothing is logged when I click Apply, as the button is just disabled by this delightful form validation.