Currently, if you are not signed in and are given a link to content within a Private (Unlisted) community (e.g. a specific forum thread/post, wiki article, etc.), the workflow when you successfully sign in is:
- If you are a member of that community, you get to the content URL
- If you are NOT a member of that community, you get a really uninformative "Post not found" message.
We may be seeing this differently than others -- if not, it would be good to know what we need to do (without customization), but if this is the current OOTB workflow, this user experience must be made better (i.e. those who are not a member of the Private (Unlisted) community are redirected to the community home page, where they can request access to the community).