I tried to install Ubuntu 18.04 as a second OS, but now neither of them is booting anymore. While boot "System BootOrder not found. Initializing defaults. Boot System." is displayed. See the screenshot below.
Boot error
I followed this guide and created two (/ and /home) partitions like in the screenshot below:
Partitions
Beause it did not work and displayed this:
grub-efi-amd64-signed failed installation /target/
I found this solution to create a third FAT32 boot/esp partition to successfully install grub and Ubuntu, but now I can't boot either of them.
Some more screenshots:
Boot menu
UEFI boot settings
Did I mess with the bootloader/MBR, and why did I need to create the esp/boot partition to install grub?
Additional information:
