After dist-upgrading to grub 2.00-20, normal boot is working and chosing entries in grub-menu is fine as well.
But if you use the special features of grub-reboot, where you can set the menuentry-number previous to next boot (as described here: https://wiki.debian.org/GrubReboot (https://wiki.debian.org/GrubReboot) ), this is not working atm. Instead the last saved entry will boot.
Now 10 hours later whithout anything new in D-U, everything is working as expected when I follow step by step the instructions. So self-healing or PEBCAK yesterday evening. I will mark this as solved unless noone else reports errors.