Since @frostschutz hasn't written his correct solution to the problem as an answer yet, I'll summarize the issue here quickly: I picked the UUID of the wrong partition. The root partition under /dev/VolumeGroup/root is not the one to be chosen, but the actual partition /dev/nvme0n1p2. Here's how to extract that UUID:

# uuid=$(blkid | grep 'crypto_LUKS' | egrep -o ' UUID="[^"]+"')
# uuid=$(echo $uuid | awk -F '=' '{ print $2 }' | tr -d '"')

Which then can be used in the boot loader entry config:

# cat <<EOF >/boot/loader/entries/arch.conf
title   Arch Linux
linux   /vmlinuz-linux
initrd  /initramfs-linux.img
options cryptdevice=UUID=${uuid}:cryptlvm root=/dev/volgrp/root
EOF

I summarized the whole procedure on my private website. Thanks also to @Cbhihe for the advice on partition sizes.

Since @frostschutz hasn't written his correct solution to the problem as an answer yet, I'll summarize the issue here quickly: I picked the UUID of the wrong partition. The root partition under /dev/VolumeGroup/root is not the one to be chosen, but the actual partition /dev/nvme0n1p2. Here's how to extract that UUID:

# uuid=$(blkid --match-tag UUID -o value /dev/nvme0n1p2)

Which then can be used in the boot loader entry config:

# cat <<EOF >/boot/loader/entries/arch.conf
title   Arch Linux
linux   /vmlinuz-linux
initrd  /initramfs-linux.img
options cryptdevice=UUID=${uuid}:cryptlvm root=/dev/volgrp/root
EOF

I summarized the whole procedure on my private website. Thanks also to @Cbhihe for the advice on partition sizes.