We found this bug by having the following problem:
We couldn't mention an specific user (doris.doppelbauer) within Confluence by using @user while creating or editing a page.
We found a Explanation (see link at the end), which isn't working in our case, because the user appears in the people directory four times, while the link to the Profile itself is always the same. The search for this user won't give aus any results.
I checked the rights and roles and in my opinion everything is alright. The user is able to watch the spaces but nobody can mention this user.
I made some screenhots in order to make you able to see the problem.
If you have any further questions, please go ahead and ask me. Please fix this issue as soon as possible, as we need to mention users for our workflow.