@ToshM: What happens when you attempt to move photos to your Secure Folder. Do you get any error messages? Can you confirm if you have attempted to move the photos directly via the Secure Folder, and by using the 'Move To Secure Folder' option on the photos themselves?
1) Open the Secure Folder > Add Files (3 dots > Add Files) > Images > Select the images you want to move > Done > Choose whether you want to copy or move them. To troubleshoot the issue, try both options at this stage to see if the problem persists.
2) Open the Gallery app > Select an image > Tap the 3 dots > Move To Secure Folder.
Let me know how you get on.
@ToshM: Can you try method 1, as provided in my previous post, to see if you're able to move the photos? I assume you are attempting to move multiple images at once, therefore you may wish to try moving 1 at a time as a process of elimination, just in case there is an issue with one or more of the files.
I am having the same issue, wherein unable to move photos and getting an error "couldn't move items. 0/1 item moved"