The way I originally generated the link to that folder was incorrect (I copied it from the web browser address bar). Having done it properly (using the article you linked to) has worked fine.

Thanks. I've just rechecked it and not sure how it happened but, as you said, it lets me save it. I'm not sure what happened. It might have been something to do with permissions.

I think it might be something to do with my url. I'll try to come up with a way of generating the link for a group given access via direct access somehow. I'll report back.

Thanks for your reply but I'm confused. How is my 'hierarchy of subfolders' different from 'different folders'. Both, as any folder structure, are really a hierarchy of (sub)folders. Unless, I've misunderstood your answer:)

I've stopped inheriting permission at the document library level as well. Also, what's interesting if I navigate to the HR folder from the top level of the document library, it's doing fine (without any notice). If I use a link (basically a copy/paste from the browser url), then the obove notice appears.

Actually, I had done it at the folder level.

I have not done it at all. Shall I do it now at both levels?

Thanks. I've updated my question above