How Turn Off Warning When Opening External Files?

Previously, I could go to Edit > Preferences > Security (Enhanced) and Uncheck the box that said Enable security at startup. After restart, one is asked only once whether to open an external file in a little window that is provided with a check box enablng one to turn off that warning in the future. Now, however, on a new laptop that I recently bought, that process does not work. I can Uncheck the box as I just described (and restart of course, including even re-booting), and I get faced with the warning every single time that I want to open an external file. I don't know what would have caused that. I believe it may have defaulted to the new interface when I first opened it, and once I learned how to switch the interfaces  back and forth, I did so several times.But no matter the interface, the method I've used previously did not work. My choice was simply ignored by Acrobat. I don't dare experiment on the desktop where I normally work for fear that I will lose the old way forever!

Is this (I hope) just a bug in the latest version? Please help, as my readers will be opening external media files hundreds of times in the several PDFs they will download!

(I am not provide with the usual several topics to choose from.)