It's not a Kleopatra bug. His installation is most likely fine. I bet OP is, by mistake, trying to verify the installer.exe with the portable.asc file. Because both files are next to each other and similar in filenames, he downloaded the portable.asc file, and when trying to verify the installer executable you get the i/o 218136625 error. If you try by yourself, you'll see this is the same error that you get when you are trying to match the wrong file with the wrong signature. If someone else finds this thread this will be useful to be left here because no one pointed this out yet.