Ubuntu 18.04 and Windows 10 not booting after dual boot installation

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:

Secure boot configuration