Sorry, you are not allowed to share this with external users.

I have setup a site in office 365 SharePoint to share documents with clients. I have added 2 external users without problem, both of these uses have a @hotmail.com email address. I tried to add another external user with an email address @theredomain.com.au and I keep getting "Sorry, you are not allowed to share this with external users".

I have disable external sharing then re-enabled it to see if this helps, it didn't. When testing the setup I was able to add external users that had email addresses @theredomain.com.au, @gmail.com, etc. But now I can't what is the go?

We are in Australia on the Small Business Premium plan (P2).

This is important and need it resolved now.