Thanks Jack,
So on the 2nd upload to browser, the file is loading ok?
The issue you described with the first upload not working is a known issue (see point 2.1 in my previous comment). The skp 2021 converter is in TCWindows only for now, so on the first time uploading to the server - the backend will create a TRB file but this will fail because the processing engine on the server does not have support for 2021 yet. The second time you upload the SKP file the trb file from TCWindows will replace the failed file on the server.