Hi,
grub seemed to be the main issue. Bootrec.exe /FixMbr and Bootrec.exe /FixBoot didn't worked for me since the grub didn't uninstall.
Bootrec.exe /FixBoot gave also the message "bootrec fixboot element not found" (used the Win 7 installation CD to boot from)
(http://www.howtogeek.com/howto/32523/how-to-manually-repair-windows-7-boot-loader-problems/)
I followed some additional steps:
I used the Diskpart tool commands:
where I typed the following
DiskPart = starts partition manager
List Disk = list all hard drives
Select Disk # = usually 0 like Disk 0
List Volume = lists all partitions
Select Volume = usually select volume 2
Active = marks partition as active
exit
Here some additional info
https://social.technet.microsoft.com/Forums/Sharepoint/en-US/7791044e-db7f-4144-a96c-945299811f58/bootmgr-not-found-bootrec-rebuildbcd-and-fixboot-give-element-not-found-error?forum=w7itproinstall
(https://www.youtube.com/watch?v=KV84OabGB08)
Then I got the message "Bootmgr is missing Press Ctrl+Alt+Del to restart" and booted again from the Win 7 installation CD and then followed the repair option as described here:
https://support.microsoft.com/en-us/kb/2622803
(https://www.youtube.com/watch?v=MmAbyGFSatg)
Finally the system booted from windows bootloader and the upgrade from Win 7 to Win 10 was done successfully.
Thanks for the tips!