Hello Pascal
I tried the installation with the current installer from testing.
I assume, the installation was successfully with testing, because there
is grub-installer 1.214.
In changelog to 1.215, there was Bug #1138519 solved:
[ Pascal Hambourg ]
* Always mount sysfs in the target filesystem (Closes: #1138519).
I don't know, but possibly there are some problems with the efivars.
There is also bug #1144078:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1144078
Also problems with grub-installer.
One day after upload 1.215 to unstable.
These are all assumptions, but possibly helps a little.
If you need more informations, log files or other stuff, please let me know.
Best regards
Bernhard
