Hi there @AndyCooper66! Sorry for the late response, and thanks for sharing your findings. I'm glad to hear there are some workarounds for now.
New updates have recently been released for Lightroom Classic and Photoshop. Could you update and test if the behavior is any better?
When you respond, please mention my username with an @ so I get a notification and can get back to you faster.
Thank you!
Alek
No problem Alex. I have further news and possibly a solution of sorts.
Even if Photoshop opened the files, it would not save the files with an "Error 2" problem. These problems remained when after the recent updates.
Investigating this issue asking with my problem came to the only solution that worked. I removed all plugins, external editors, Lightroom Classic and Photoshop including all preferences.
I then installed Photoshop. Once completed, I opened it to ensure it was installed. I fully closed it and then I installed Lightroom Classic. This has sorted the problem - I've tested many files and all seems alright. What will happen as plugins and external editors or updated are added is yet to be determined.
Hope this helps someone.