It has been awhile since I have used MEPIS, but it has a utility in the KDE
system menu to rewrite grub. If it did not do it properly the first time
then it isn't likely to do it again, but it is worth a shot.

Otherwise, you will have to cut and paste from the menu.lst in /boot/grub of
your Ubuntu installation to your menu.lst in Mepis. You will need to use su
in Mepis to load its menu.lst found in the root directory at /boot/grub. I
think that Mepis has a file manager in administration mode in the system
menu. It used to. Use that instead or use a terminal and change to root with
su and then launch konqueror. Navigate to /boot/grub/ (in the MEPIS root
directory) and open the menu.lst in kate.

Next open the Ubuntu menu.lst. You don't have to be root so use an ordinary
file manager. Open it in kate. Copy the lines from the Ubuntu menu.lst that
you need. If you place them at the top, ahead of MEPIS, then Ubuntu will
become the default boot OS. Otherwise paste it after the MEPIS lines. Save.

Possible problem. If you have separate drives and use a boot manager to
choose which drive to boot first and both drives are set to hd(0,x) (mine
work this way, but most people's don't) then you will have to edit those
lines of menu.lst to reflect the change. Basically if they are on the the
same drive it should be no problem, but if they are on different drives then
you may have a problem when you reboot. If so you will have to manually edit
grub at boot up (press e, change and then press b to boot) and then make
changes when you get back in.

this is probably similar
https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows

Thank you very much ! I looked on the menu.lst but couldn't find anything
wrong, Ubuntu was also listed there but just could not be booted. I booted
Macpup Foxy and deleted GRUB from the Ubuntu partition but this did not solve
the problem and then unplugged the IDE hard disk to work on the SATA one, booted
the Ubuntu live CD and restored GRUB according to the instructions on the
Internet but this did not solve the problem either, Kanotix's GRUB loaded, with
Ubuntu in the boot menu but with the same error when trying to load it. As I was
desperate, I booted from the Kubuntu live CD, started the installer, resized the
Kanotix partition and installed Kubuntu. Its GRUB now loads Kubuntu, Kanotix or
Ubuntu with no trouble. It seems that there is really something wrong with
Kanotix's installer. Now I have a three-boot system on the giant SATA drive,
with Ubuntu and Kanotix on two huge partitions and Kubuntu on a smaller one and,
independently, another
three-boot system on the IDE drive, with gOS, Linux Mint (on most of the space)
and Macpup Foxy frugal. I did not really want this, but at least I have six
options.
I might seem to be an undecided maniac.