Thanks @Katya and all. I sent this in to connect-support and recieved an email back from them this morinig. They confirm that the current edition of Trimble Connect 3D viewer cannot open SketchUp21. Apparently the SketchUp plugin for the 3D viewer has not been updated. They created a ticket TCS-3744 for this issue. Their workaround suggestion was to use Trimble Connect for Windows app to view models until the issue is resolved, which being on a Mac is a non starter for me. An easier workaround I think is to back-save files to 20 for now.
At least attention is brought to this issue and let’s hope the 3D viewer gets fixed soon.