My WiiU runs v5.5.0.
My 16GB SD card kept failing when launching Loadiine with the message "SD Mount Failed". I looked into the homebrew apps you're to copy into the SD card and noticed some are kind of old.
I went ahead and download the very latest builds of the Homebrew Launcher, ddd and Loadiine, replacing the files already there. I didn't modify the htdocs from the zip file at all.
Now, I'm not 100% sure that was the reason it started working again, it could have also been that I used SDFormatter instead of Windows 10's stock formatter to reformat my SD to FAT32 with a cluster size of 32KB (picked for me).
Whichever one it was, now it works. I hope this helps someone who's struggling!
Here's exactly what I used:
Self Hosting.zip CRC32: 07C3712A
ddd.v0.3.zip
homebrew_launcher.v1.2_RC3.zip
loadiine_gx2_7e4313d.zip