oh, what a wonderful morning...
while getting up I saw your reply tried immediately and...
it worked!!
there was no line to change /dev/sda7 to /dev/sda6,
it nevertheless bootet into ubuntu.
after sudo update-grub no boot problems anymore.

Are there tutorials to learn about the "inside" of ubuntu and the terminal commands to influence it?

your help was very generous for I knew my problem was a bit off topic of this forum. Thanks.