What does 'sp_configure allow updates' do?
This is an old option from the SQL Server 2000 days that would allow you to directly make updates to the system catalog tables (sysobjects, syscomments, etc.). Since 2005, this is no longer possible (at least not without jumping through a ton of additional hoops beyond just an sp_configure option - it's not something you ever want to do on a production system), and all of the system catalog is now exposed through read-only views like sys.objects.

I'm not sure how that would be related to the maintenance plan failing, though. That error you saw is pretty typical if you delete or otherwise remove a database that's selected as part of the maintenance plan. Usually, editing the offending step in the maintenance plan and resaving is enough to fix it.