@baz Have you ruled out an extension clash by only loading the 3Dx extension?
Judging the ruby output SketchUp's own extensions.rb is complaining
There is a big difference in how SketchUp handles webdialog stuff between 2019 and more modern editions. Recent changes did break older approaches.
If it was me, I'd rename the Plugins to ###Plugins to prevent it loading so I would have a vanilla SketchUp install.
See if that still throws errors.