"Lower Security Preferences" opening AVCHD files in Quicktime X

[Jimmy Pierce]

This started happening recently after upgrading OS. When I try to open an AVCHD (subfolder), Quicktime opens, thumbnails of clips appear (as always), but when I select one and click "OK", a message pops up saying to "lower security preferences for Quicktime". I clicked "lower security" or whatever the option was other than "CANCEL", it failed, now I get an unknown error message when I try again. Looking security preferences, don't see what I need to adjust to fix it. I realize I can just right click on the AVCHD folder, go to the STREAM folder, etc or look at them via editing software, I just like the convenience of Quicktime.

Suggestions or solutions appreciated.

UPDATE: Found an Apple thread circa 2017, no solution, though.
https://discussions.apple.com/thread/8110653