Butler generates a signed session and sends your file directly to Google Cloud Storage, not going through our servers. The only thing I can imagine is that the network connect between you and the Google Cloud Storage endpoint is being blocked in some way. Network issues can sometimes be regional. If you have access to another computer it might be worth trying that as well, in case it’s not the network, but the computer itself.
I’m not currently aware of any other reports of people having issues like this.
Hope that helps you diagnose the issue