I've taken it down for now. No idea why that happened right now. But I'll get this fixed first thing tomorrow. In the meantime, you could try just running this command: ./SpacePad_v1.AppImage --appimage-extract-and-run.... it's possible there's a mint compatibility thing I need to look into, and if there is, this should let you use the app in the meantime.
OK, this was a doozy. I uploaded a new a super solid appimage that works in mint 21.3, full instructions in the readme. If you're using a high density retina display, the window might be significantly smaller than designed. Spent all day trouble shooting it. But a full fix for window scaling will have to wait till v1.2. If you're using a 1920x1080, display, should be perfectly fine!