Wordpress warns users when two of them are editing the same article, but the warning always says "admin is editing this page". Admin is currently logged off and the message is caused by another author.
Language files have the %s in place.
There are two separate post meta values related to this behavior:
_edit_lock- holds timestamp when post was last locked;
_edit_last- hold ID of user who last edited post.
Most likely these somehow got out of sync - lock was triggered recently, but user wasn't properly updated. Hard to guess why from information you provided.