The laptop is a recent Dell XPS 15z and the problem is as follows (reproducible about 90% of tries):

• put laptop to sleep using either Start->Sleep or closing the lid
• laptop goes to sleep after about 5 seconds, but instantly wakes again showing a black screen (touching the keyboard or moving the mouse shows the login screen one normally gets after wake)
• login again, put laptop to sleep
• latop stays in sleep mode

output of powercfg -lastwake after the first instant wake shows the audio controller is responsible. Why would that be, why only the first try, and how to fix this?

Wake History Count - 1
Wake History [0]
  Wake Source Count - 1
  Wake Source [0]
    Type: Device
    Instance Path: PCI\VEN_8086&DEV_1C20&SUBSYS_04461028&REV_05\3&11583659&0&D8
    Friendly Name:
    Description: High Definition Audio Controller
    Manufacturer: Microsoft