Apple - Gatekeeper preventing from opening files from Finder (launching apps is fine)

Just experienced a similar issue on a 10.13 system after having migrated it from a 10.12 system -- turns out /var/db/gkopaque.bundle wasn't valid, so I copied that from a good 10.13 system (by making a tarball as root on the good system, extracting it as root on the destination system), restarting (the system or killing syspolicyd), and that fixed the issue.

As others have said, replacing /var/db/gkopaque.bundle worked for me. In my case I pulled it from a Time Machine backup by:

• Running: $ defaults write com.apple.finder AppleShowAllFiles TRUE
• Relaunching the Finder (command + option + escape)
• Navigating to /var/db/
• Entering the Time Machine and navigating to an older version of gkopaque.bundle and clicking restore
• Exiting the Time Machine
• Rebooting
• Running (to confirm): $ spctl --assess -vvv --raw /bin/ls
• Running: defaults write com.apple.finder AppleShowAllFiles FALSE
• Relaunching the Finder (command + option + escape)