What does [298] SQLServer Error: 15404, Could not obtain information about Windows NT group/user 'WIN-443PT0LULQU\Administrator', error code 0x534
Okay - after struggling with this for a couple of days (I had tried posting variations on this questions to stackoverflow and to superuser), it seems to be that the SQL Instance had the wrong machine name stored in master - so I had to change that to the correct current machine, and ensure that the correct admin account was enabled as an sa account.

This mostly got me going:
http://www.sqlservercentral.com/articles/SQLServer/135372/

I had to do run these to figure out that the instance had the wrong servername, then to drop the bad one, then to add the correct one:

use master
select SERVERPROPERTY('servername')
select @@SERVERNAME
select SERVERPROPERTY('machinename')
go

use master
exec sp_dropserver "WIN-443PT0LULQU\SOURCEGEARVAULT"
go

use master
exec sp_addserver "BUILD-SERVER","local"
go


I also had to mess around with logins - but I cannot say for sure what I did exactly that ended up working.  I renamed "WIN-443PT0LULQU\Administrator" to "BUILD-SERVER\Administrator" at one point, and after doing the above, I had to restart the instance.

After that, things seem to be working properly, and I am able to execute my maintenance plan successfully.