Settings contains a dot which is not supported, but module gets enabled properly
Okay, classical scenario, 30 seconds after I open a bounty for the question, I find the answer myself ...

I was looking into the wrong file. I started the skeleton of my module by following this tutorial. 

In the "Basic Structure" section of the tutorial, dots are used in keys for the .schema.yml file. It is actually forbidden to use that, but the tutorial said you could anyway. 

/config/schema/loremipsum.schema.yml :

loremipsum.settings:
  type: config_object
  label: 'Lorem Ipsum settings'
  mapping:
    loremipsum:
      type: mapping
      mapping:
        page_title:
          type: text
          label: 'Lorem ipsum generator page title:'
        source_text:
          type: text
          label: 'Source text for lorem ipsum generation:'


I removed the dot at the key, and the problem disappeared.