Make sure the "Search users" permission is given to every user who is allowed to post. This permission is necessary to be able to search users after tying @.
I think it unfortunately also performs the search when you quote a post using the direct button so the permission is still needed even if you are not manually searching for users.
We should probably silence this error in the future since it's perfectly possible to use mentions even without the ability to search users, the preview might just be unable to render correctly but after posting it will be fine.
