Sagemcom F@ST 3864AC, CFE error 18/21

Hi, I'm currently trying to get OpenWrt to run on my telecom-provided router. I've gotten into the CFE via a physical serial port (the password is $agem001 for those wondering) and am trying to use an OpenWrt ramdisk image built for a similar router using the same CPU (BCM96318) over tftp, but CFE refuses to boot with either an error 18 or 21, neither being explained.I would attempt to flash an image, but being the locked down router it is, the default firmware is not available online, therefore a bad flash would render it useless unless I get OpenWrt working. Anyone experienced these errors before?

Did you manage to find a solution ? I have the same router and looking for a custom firmware to flash