The bug is squashedjamesbond wrote: EDIT: here is the fixed split-initrd. Please give it a try and see if that works for you.
I ran your script without errors, the big initrd got split into a small initrd and the fd64-620.sfs, I copied them to the root of the sdcard and rebooted. Entered the passphrase of the fd64save_dmcrypt_.ext4, no errors this time. I changed the wallpaper, rebooted and the changed wallpaper was still there, so the savefile keeps the changes. It works.
Thanks for the excellent guidance!