The problem is/was caused by the presence of Rockchip files in the JSON data that the USB/SD Creator (and settings add-on) client read to populate the list of available files. The Rockchip files are using a slightly different file naming convention and this upsets the client. The temporary workaround is to rebuild the JSON file without the Rockchip files - which has been done, and a quick test with the Win/macOS/Linux versions shows the app(s) are working okay again.
Confirmed on Win7. I just re-downloaded the EXE and it appears fine now. Thanks for your help!!!