SSIS - This error is thrown by Connections collection when the specific connection element is not found
The problem lies in the referencing. You might have copied the package from an older one and are making the changes or you might have edited the connection managers. The connection managers are internally used using the GUID that you have mentioned. You might have changed most of the references from the UI, but still there are some objects and tasks, that are referencing the deleted or older connection managers. The probable one would be the SQL Server Logging in the package.

Solution is: Open the code of the package, by right clicking the package in the solution explorer and clicking on View Code which shows the package code in XML format. Now you are able to see, which tasks are referencing the old connection manager. Do the needful changes and this should do it! 

Let me know if the problem persists after this!!