Tested a lot of stuff and found following behaviour:
When I add a content type to authentication an create core routes and core controllers, the documentation plugin then generates a doc for these.
My custom routes in an additional custom-routes.js within the routes folder are still ignored by the documentation but the error is not ocurring anymore.
Is it true that the current version of the documentation plugin can only document core routes?
I cant find any information on this on the plugins info page.