Multiboot with Ubuntu : invalid signature

[nhp...@gmail.com]

After 6 hours of trials, I finally to search for help here:

I've installed Qubes onto my PC with Ubuntu and Win10 already installed.

To multiboot Ubuntu with Qubes (Win10 is unimportant here), I followed the Multiboot tutorial and handled with some modifications of the parameters without luck.

After some trials on Qubes Bootloader I re-installed the Bootloader of Ubuntu and added the commands into
/etc/grub.d/40_custom.

I always get the invalid signature message

/dev/sdb1: UUID="7EF489BEF48978E3" TYPE="ntfs" PARTLABEL="Basic data partition" PARTUUID="e9b9258c-eb19-4917-ba4b-6be9b6120063"
/dev/sdb2: UUID="2A56-C781" TYPE="vfat" PARTLABEL="EFI system partition" PARTUUID="e509094a-abf7-4602-a6a5-12cb27f926a3"
/dev/sdb4: LABEL="Win10" UUID="2ACC640FCC63D41B" TYPE="ntfs" PARTLABEL="Basic data partition" PARTUUID="d431cf1e-218d-44ef-a592-8721bb12ae96"
/dev/sdb5: UUID="53c7d5ce-1628-4fff-b764-96ba2e9cfc7e" TYPE="ext4" PARTUUID="219f1f65-1156-4489-9075-609208d4d268"
/dev/sdb7: UUID="62c8211a-9a75-4f03-b19d-a42144b2b811" TYPE="ext4" PARTUUID="14457fbc-1dec-476b-8d00-f25ab8781024"
/dev/sdb8: UUID="phAkAt-gqMO-jgfU-ymJ5-c2ig-B3Rw-A7SO8h" TYPE="LVM2_member" PARTUUID="eadaa2a5-5e9e-4563-9ff5-02f567a36036"

Triet event to boot Qubes through the entry:
insmod part_gpt
insmod ext2
set root='(hd1,7)'
search --no-floppy --fs-uuid --set 62c8211a-9a75-4f03-b19d-a42144b2b811
chainloader +1

Need some help in solving this confusing situation.

BTW = The sdb7 folder has an EFI folder
My Ubuntu x64 16.04 doesn't

I can reinstall Qubes if needed, but I am unable to delete my Ubuntu partition, needed for my daily work.

[cooloutac]

maybe you have to turn off secure boot in bios?