Before installing with the offline installer, rename the "Opera GX Stable" folder to "Opera GX Stable old" just to see if that sorts things out. If not, totally uninstall Bitdefender (don't just disable it) as a test and then try to install Opera to test if Bitdefender is the cause. You can install Bitdefender again then and if it was the problem, see if you can add exceptions for Opera, its profile and cache folders, and Opera's install folder etc.