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.