macOS High Sierra - all documents damaged after migration from another mac

I've migrated just about everything from my MacBook Pro running Sierra to another MacBook Pro running High Sierra upgraded from a clean El Capitan.

As a result - all my (migrated) documents, as well as newly downloaded ones, when I try to open any of them - are reported as damaged and should be moved to trash(!?)

Actually - the problem happens even with the brand new file I've just created:

I understood that this has to do with SecAssessment system policy security (spctl), and disabling it actually helps, but I think there is a point to having it enabled, and it should make sense somehow rather than declaring all my content - damaged.