"You can’t change the item to always open in the selected application." OSX thinks .jpg is an app from an 'unidentified developer'

I have only just upgraded from a MacBook Pro 13" Late-2013 to a brand new MacBook Pro 15" (2018). Bluetooth problems aside, I now noticed that many of my .jpg-Files cannot be opened in Photoshop via right-mouse-click and the drop-down menu "Open With... (Photoshop)". This is because there is no selection for Photoshop anymore, as the OSX believes the .jpgs (nice ones from professional image platforms and large publishers) to be apps. So it only gives me either Preview as the default, or else the Terminal.

I'm a graphic designer and have a huge archive of photos I need to constantly use. Double-clicking on the JPGs will open them in Preview without any problem whatsoever. I just cannot drag-and-drop them into Photoshop anymore. The only way to get them to open via the right-click menu in Finder is to go via

"Open With..." -> "Other" -> double-click the Photoshop CC19 Folder

Then the PS app icon is greyed out. I need to click on 'Enable Recommended Apps' and change that to 'All applications'. I can then also tick the box 'Always Open With'.

Once I click 'Open', the image will open in Photoshop. BUT at the same time, I get an error box, telling me that

You can’t change the item “image_name_here.jpg” to always open in the selected application.

The item is either locked or damaged, or in a folder you don’t have permission to modify (error code -10823).

So basically, the OSX believes a JPG it opens in Preview without any ado to be an App.

There was a thread on the topic over here:

https://discussions.apple.com/thread/6664136

but it hasn't seemed to have resolved the issue.

Why has this problem only just occured now, with a brand new MBP, and with a collection of imagery that I have never had a problem with prior? As said, it only affects about 50% of my library, seemingly randomly.

Any help much appreciated, working is very difficult like that.