Can't edit maintenance plans after upgrade to SQL Server 2016

Hi, We recently upgraded our test Environments from SQL Server 2012 to SQL Server 2016 (SP1). After the upgrade we get en error whenever we try editing Maintenance Plans. I can open Maintenance Plans from Management Studio (13.0.16106.4) but non of the tasks are editable. Renaming a maintenance plan from the GUI gives me the error "Login failed for user 'sa' (Microsoft.SqlServer.IntegrationServices.ObjectExplorerExtensions)". Deleting a plan and recreating it works, and renaming a plan I created after the upgrade works fine. Also, in the GUI, when I open a task, I can change properties in the properties window, but I can't edit the task from the popup window. The OK button is disabled. Did anyone else experience the same thing (and yes I know, Maintenance plans are..... But now we have them...)