Sharing is no longer working on my hosted nextcloud service

I have a Nextcloud account with one of Nextcloud’s preferred hosts. I can’t share any folders as of a few weeks ago. Whenever I click on the name to share with it says

“Sharing {name of folder to share} failed, could not find {name of person I’m trying to share with}, maybe the server is currently unreachable or uses a self-signed certificate.”

The other person also has a Nextcloud account with the same host (if that makes a difference). I’ve tried several different people to share with, the result is always the same.

Is sharing like this no longer possible?

In nextcloud there are some sharing and finding people option. Check them. Are you admin? Do you have an own nextcloud?

Done that. As I said.

If there are some other options, I’d be grateful if you could point them out.

No. As I said.

Can you test sharing with other clouds. If user@cloud.domain.tld is there shown nothing or the mail icon? If mail icon test with a slash at the end e.g. user@cloud.domain.tld/

With hoster do you use?

I’m sharing by using the email addresses that the people used to sign up to the host. It’s the only way I know how to do it, I’m not sure how to test sharing with other clouds I’m afraid.

Oddly, when I first try to share a newly made folder it says

“Sharing is only allowed with group members”

Then the second time I try to share (exactly the same folder, exactly the same person), it brings up the message I quoted above.

I should also add these are people I’ve shared folders with before using exactly this method which is now not working.

When I start to type in the email address, the contact shows up in a dropdown list below the text entry box. I click on their name and two options turn up - one to share, the other to email a link. If I click on the option to share, nothing happens except the error message I quoted above. If I click on mail it sends the person an email and they can access the folder exactly as expected. It’s just sharing that’s not working.

Another element I’ve just discovered. If I go to sharing options on a folder which is already shared and change one of the settings (try to click on ‘Can Edit’) it says

" Error whilst sharing. Sharing is only allowed with group members"

This is folder which is already being shared.

What is a ‘group member’ in this context?