I already discussed it on the #wiihelp channel, but I wanted to know if someone has the same problem:

History:
Downgraded from 3.3u to 3.2u with cIOS Downgrader long ago, tried to install the modified mios... it shows the screen above.
Downgraded from 3.2u to 2.2u with the same program... tried to install... no luck.
Updated with mariokart to 3.1u and tried to install, no luck either.
Updated to 3.2u with any region changer, no luck.

It seems that you can install the mios modified using the wad installer... is there anyone who likes to share the modified mios with me? thanks in advance.

I'm sorry but unfortunately I'm pretty sure that would fall under copyrighted code so you'll have to look else where.

Look at the thread that I just made.