Thanks Yury for this advice. A restore to the BIOS defaults did the trick. A restore to the factory defaults did not help.
On my XPS 13 9360 DE I got an notice from the Ubuntu Software Centre to update the BIOS. So I confirmed to perform the upgrade. After a reboot I saw messages about flashing the hardware. After the next reboot and all others I get the message: No bootable devices found. Before this message, following message is quickly shown: [IMAGE]  Does anyone have the same issue? Any ideas how to proceed? Thanks! (ePSA: Build 4304.14 UEFI ROM)
BIOS upgrade 1.3.5 => No bootable devices found.