Hello, I downloaded firmware-9.6.0-amd64-netinst.iso and successfully installed a minimal debian stretch on my UEFI/GPT disk. After rebooting, I was presented with the grub2 text user interface and the possibility to boot Debian or my Windows boot manager - as expected.
Following
https://wiki.debian.org/DebianTesting, I edited /etc/apt/sources.list and changed stretch to testing, apt update && apt upgrade. Everything seemed ok but when I rebooted, the grub2 user interface was gone and I could not find a way to boot Debian. Any ideas?