Error 0xC0012050 while loading package file

When a parent package executes a child package which has a connection the following error may occur:

Error 0xC0012050 while loading package file “Child.dtsx”. Package failed validation from the
ExecutePackage task. The package cannot run.

Resolution:
Make sure that DelayValidation is set to false on the child-package definition (the package itself, not the ExecutePackageTask).