I'm also having this issue. I run multiple rpis with LibreElec 9.x, including rpi1 (no LE10 image) and rpi4 and am also using Hyperion (does not work on LE10) so really want to stick with 9.x.
Just got a RPi4 2Gb to upgrade one of my systems but am getting this issue.
Tried copying start.elf from LE10 but this is not working.. you mention another file that needs to be copied.. any idea?
edit:
Tried copying start.elf and fixup.dat as mentioned in other threads but I get Invalid ELF header: 'start.elf'
Thanks!