In Magento 2, all dependency injection settings are now created in a di.xml file. All XML data in the new Magento version must now be validated against an XSD file. This happens both at runtime and in modern IDEs such as PHPStorm. Automatic code completion is equally useful.
PHPStorm reported the following to me:
The type definition cannot be abstract for element argument
In addition, code completion stopped working in this case.
The solution was quite simple. I had forgotten to specify the xsi:type attribute. This attribute defines the type of the argument ("string" in my case).