Comment 47 for bug 1835660

I had also the same message in dmesg. (I don't remember when it started)
Here : Ubuntu 19.10 booting a manually installed kernel (5.5.2-050502-generic)

I followed previous advices :

Adding COMPRESS=gzip to /etc/initramfs-tools/initramfs.conf (instead of lz4) then :
$ sudo update-grub;sudo update-initramfs -c -k all;sudo update-grub
Reboot.
No more error message, and systemd-analyse says :
Startup finished in 15.790s (firmware) + 3.112s (loader) + 9.986s (kernel) + 9.605s (userspace) = 38.495s
graphical.target reached after 9.595s in userspace

I restored COMPRESS=lz4 then
$ sudo update-grub;sudo update-initramfs -c -k all;sudo update-grub
Reboot : still no error message, and systemd-analyse says :
Startup finished in 15.793s (firmware) + 3.188s (loader) + 8.962s (kernel) + 11.645s (userspace) = 39.589s
graphical.target reached after 11.635s in userspace
(Yes it's slower but it might be me : I have to choose in grub menu selection and type full disk encryption password)

I rebooted a 3rd time : no change, still no more Decoding errors.