There might be some remainders of an old installation of VirtualBox. This is the least likely.
The downloaded file might be corrupt. This is the most likely. Re-downloading and re-rpm’ing gives you the most changes of success.
The partition on which the rpm or system got stored might contain physical or logical errors. Using the command ‘dmesg’ tells you whether or not such errors have taken place. Not so likely, but possible.
VBox additions is as you know to install on the guest. The .iso should be in /usr/share/virtualbox
If there is a problem with the .iso file, you will find that out when you need to use it, you can always download it separately I would think. Or re-download the .rpm and run Uhv from the terminal CLI to update.