This is what happens. I have no idea why this happens.
https://gyazo.com/e47fc225f840c041193122a4c7ef5294
ERROR
Details: reader error
An unrecoverable fault has occurred, please report the error by visiting the website below
http://powdertoy.co.uk
Yes, I have reinstalled it from scratch twice. This does happen every time. I get a Memory Read/Write error.
I have also tried it with relocating the Powdertoy app on my PC, and also running it as Administrator. Both resulted in the same error.
When you reinstalled, did you reinstall to a completely different folder, and try it with nothing else in it (Copying none of your stamps or saves, or powder.pref)?
nothing, or an error? If you get literally nothing, then that's a different issue.