You broke my heart when you said you skipped (re)installing priiloader, this is uninstalled when installing a new system menu, which is stated multiple times in your modmii guide. I didn't even finish reading, but before u do anything else, reinstall priiloader!!!
Edit: you know I never considered this before as part of the region change guide... surprisingly, I guess no one ever commented on it before. So thank you for bringing (the following) to my attention
Your wii channels that are not region free (I. E mii channel) are still the region you had before. You would need to very carefully uninstall these (or leave them), then install the channels for your new U system menu version. See modmii classic's download page 1.
To uninstall your old channels u will need to do some steps that could result in brick if u do not know what u are doing. As long as you follow everything I'm going to say carefully you will be fine. There's 2 (or more) ways to accomplish this, first, just carefully use some yawmm mod to uninstall ONLY the old Jap Wii channels/wads, which u may have left over if u used ModMii to softmod the console when it was still J. Or another way which doesn't involve sourcing the wads is to use a homebrew app called any title deleter to delete ONLY the old J channels that are broken now that you are on U.
Edit2: don't act rashly and do anything raxadian suggested! First thing, install priiloader. 2nd thing, create and share a Syscheck log for us to see.
3rd thing, probably what I said above about installing new channels for your new shiny firmware. Some of these channels don't work anymore, like news and weather (due to lack of Nintendo service). For that you want to use RiiConnect24 to install their IOS31 and the channel in question, and an app for mail, etc. Basically do whatever they suggest but skip installing their IOS80 if you've already used ModMii 6.6.0 or higher (which you can tell by sharing a syscheck log)
Edit3: If u remove the wrong title with with any title deleter you can brick! If u uninstall the wrong wad you can brick (same goes for installing)! I said to use some yawmm mod specifically because it will prevent u from doing potentially catastrophic things like uninstalling your system menu or system menu IOS. So if you ever decide to even attempt to uninstall a wad, you should not use MMM or another WAD manager, you should use Some YAWMM Mod.
, thanks for editing your post with my quote to suggest he reinstall priiloader first
God I hope this is my last edit in this message