I know I have another command in GRUB_CMDLINE_LINUX:GRUB_CMDLINE_LINUX="... cryptdevice=UUID=:lvm root=/dev/mapper/MyStorage-rootvol ..."
Hopefully this will be enough.
Regarding partitioning, it's very standard:I encrypted the whole thing, then I partitioned it as one / ext4 partition, and a swap one. As long as you don't have a dedicated /boot partition, it shoulldn't be a problem.
I never had this error though, this deserves some more investigation.
