Error 9934 in Sierra?

I have just successfully transferred some photos using Image Capture from my iPhone 6 (iOS 11.2.6) to my Pictures folder on my MacBook Pro under Sierra 10.12.6 which included a fairly massive "security" update which is frankly intrusive and disagreeable. Now I try to transfer some more pictures to a different folder which already contains some pictures from other sources and it responds:

The selected folder is not writable (-9934).

An error occured while importing. xx items were not imported:

there follows a list of images not imported and a box saying "OK" which of course it is not.

However I can import them into a new folder on the Mac and then transfer them from the new folder into the one I first tried and which gave rise to the error message, but I had to authenticate myself as an administrator.

What on earth is going on? Has anyone any ideas?