Narrowing down the problem, TRAVERSE now adds the LIB-OP operation to all components of a system. I added a method

(defmethod perform ((operation lib-op) (c component))

  nil)

but this method gets overriden by a more specific one which is provided by asdf.lisp

(defmethod perform ((operation operation) (c source-file))

  (sysdef-error

   "~@<required method PERFORM not implemented ~

    for operation ~A, component ~A~@:>"

   (class-of operation) (class-of c)))

This method was the cause of all problems.