Can't install Opus 12 on Win 10: error 0x8000FFFF

Sorry the installer isn't working for you.

Have you checked the digital signature on the installer? Sometimes browsers don't download the complete file, and InstallShield isn't always good at noticing. The signature is a quick way to verify the download is complete.

Please make a Process Monitor log of what happens when you use the installer, and link your account, and we'll try to work out what's going wrong: