Sadly, i have no comment for this, as i had installed the homebrew channel back in 2012 when the wii u first came out, i would think many people would say the system menu firmware version being 5.5.1 has nothing to do with it, but again, i dunno. What i did was get a 2GB SD card. (I borrowed the one that came with my Nintendo 3DS) then installed the files for the brawlstaks on it, and installed it from stage builder like normal. After the update for the hackmii installer, that allowed homebrew channel to be installed. Have you tried getting the files from Wiibrew.org? I would ask if you fot it somewhere else but not sure what difference it makes, as if you run homebrew channel it auto updates.
Best i can say is if you using a micro sd card, don't use one with adaptor, use standard one, don't enable write protection (little tab on the side of it) Use the Official Sandisk sd formatter tool, using a clean format, and then try again, if it doesn't work, perhaps the sd card is defective as people reported fakes and possibilites of obtaining one especially if you got it from the internet or for very low price compared to storage capacity, is probably just curropted somehow and no way to fix. Most savebrands to use is sandisc, Kingston, Toshiba, or Lexar based on my experience. Again, i used smashstack and i got the files from wiibrew.org website links. If it doesn't help, i dunno whatelse i can offer as i just use the stuff, with no problem, i had no reason to trouble shoot and i wouldn't have much to offer. Still the possibility of the firmware being above a specific version than a ealier one is my claim even if is possible is not the reason.
Also didn't someone state is possible to install homebrew channel with wii u homebrew access? Maybe try finding how to do that instead if it still doesn't work.