Thanks for reporting Richard; this is a tricky one to debug at times. Can you please enable the site setting enable_upload_debug_mode, and hopefully you will get a more detailed error message, and if not if you go to /logs on the site it should show a more detailed message.
I know the UX around this is not ideal either. Currently you can upload 10 files and 3 might fail, but we only show the generic dialog and “that file”. At the very least I can see if I can change that dialog to at least show the file name.