My idol! 
But is it wrong for an Ubuntu system, that might add extra stuff while creating the grub config?
If you open your encrypted container as for example Encrypted using the crypttab or luksopen command then:
You would need:
linux /boot/vmlinuz-5.15.0-58-generic root=/dev/mapper/Encrypted ro quiet splash $vt_handoff
And also use the same /dev/mapper/Encrypted in your /etc/fstab …
Otherwise you are NOT using the contents inside the encrypted container as root file system…
This has nothing todo with which distro you use, it is how you mount the root filesystem that is inside an encrypted container…
Perhaps Ubuntu specific, but I can tell you with confidence that nothing is wrong/needs to change here. 
this is the UUID of the filesystem in the opened container (equivalent to /dev/mapper/xyz)
the other one is the UUID of the encrypted partition
If that is true then yea no change needed, but makes it more clear and solid vs mistakes 
Thanks to all.
I’m about to throw in the towel, but first, I’ll try some ubuntu or kde neon forums, see if I can get the tangle out.
Again, thanks to all for your help, patience and good manners!
If I manage to figure out what’s going on, I’ll let you know!
Just to keep in touch…
I can confirm it’s a KDE neon problem 
I just installed lubuntu (that uses calamares) and I’m able to recover the system from busybox with a few steps:
cryptsetup luksOpen /dev/sda1 lubuntu
mkdir /lubuntu
mount /dev/mapper/lubuntu /lubuntu
cp /lubuntu/crypto_keyfile.bin ./
umount /lubuntu
cryptsetup luksClose lubuntu
cryptsetup luksAddKey -S 1 /dev/sda1 crypto_keyfile.bin
reboot -f
and the system boots correctly
I posted on a KDE neon forum two hours ago. By the moment no answer.
PD: I tried to install Manjaro Linux on Virtualbox (kde and xcfe edtions) but after setup calamares installer crashes on both systems but this is another topic.