SQL Server 2008 R2 Error 15401 when trying to add a domain user
Solution 1:
Old question but I had the same issue today on SQL Server 2012 (web) when trying to create a login for a domain user, something I've done numerous times before. The issue for me turned out to be the SQL Server agent which had not started up following an overnight server reboot.
Solution 2:
I had the same issue and it occurred because I just added a group in AD and was now trying to add it in SQL Server. I suspected that the Active Directory information had not yet propagated. After forcing it to propagate, I could add my group without error.