I have a Dell XPS 13 (9350) connected to an external Dell U3415W display via the Dell USB Type-C to HDMI/VGA/Ethernet/USB adapter (470-ABQN) using an HDMI cable.

When I power on the laptop and let it boot with the laptop lid open, all is fine. Both the external display and the laptop screen show the desktop.

However, when I immediately close the laptop lid after pressing the Power button, it also boots up OK with the external display showing the login screen, but the laptop front LED starts flashing 7x white, 2x amber (and repeats that pattern). That seems like an error code to me.

The Dell tech support line seems to have NO information on this LED flashing behavior. The otherwise helpful tech support girl tried her best telling me about battery charging and discharging status, but that's not what this is. 

Does anyone have any information on front LED error codes? Thx!