Try sAMAccountName is omniauth nickname is nil

lib/gitlab/ldap/user.rb

@@ -86,7 +86,11 @@ module Gitlab
         end
 
         def username
-          auth.info.nickname.to_s.force_encoding("utf-8")
+          (auth.info.nickname || samaccountname).to_s.force_encoding("utf-8")
+        end
+
+        def samaccountname
+          (auth.extra[:raw_info][:samaccountname] || []).first
         end
 
         def provider