Hello,
I ran into the same issue.
What I did was the following:
1. Downloaded the newest version of 5isfshax.ipx
2. Downloaded the newest version of 00core.ipx
3. Place both files in your SD card's hax/ios_plugins directory
4. Boot with the SD in place and make sure updates are unblocked
5. Run haxcopy and follow the prompts. It should install the files needed to the SLC
6. Boot without the SD card installed.
7. Connect to the internet and run the system update. It should actually work this time.
I was frustrated and spent hours on this, so hopefully this helps you, _Miharu_.
(I am a new member, so I can't post direct links)