Hi, This issue is now solved.
I wondered why Debian's GRUB doesn't detect Ubuntu, but Ubuntu's GRUB detects Debian at #ubuntu-fi-offtopic on freenode and I got answer that Debian doesn't have os-prober. I noticed that that package is available on Debian repositories, installed it and ran "os-prober" and "update-grub" as root and then GRUB found Ubuntu.
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