SSIS Error while loading a package: "The connection xxx is not found"

When opining a DTSX file with a SSIS (SQL Server Integration Services) package, the following message appeared:

Error 1
Error loading XXX.dtsx: The connection "{XXX}" is not found. This error is thrown by Connections collection when the specific connection element is not found.

However, none of the executables inside the package showed a red error sign and thus the big question was: WTF?

It turns out that the package was configured to use SQL Server Logging and the connection used for this log was removed!

So if you ever came across this error have a look at the package Logging or Configurations.