FWIW, I have a similar issue: A windows build I have been testing suddenly started building only under linux today, but it was starting a windows build environment as of yesterday evening.
The only difference I notice is that the failing one has this weird api box on the view config screen, and that the "dist": "bionic" line in the build config has moved. Maybe a botched or premature update?
Sorry for the troubles. There was an issue with the default-value injection scheme that broke Windows builds for a little while. This has been rectified.