I swear you guys make my head hurt.
Mine too!
And lastly, it bears repeating that custom content can cause problems too even if it hasn't in the past.
Actually this is probably the info I needed, I have to keep on testing still. That's why I asked where else can the conflicting mod be, although it seems it's custom content in the end, not a mod. Since I do have the framework installed the new way (yes, I got the installer to make absolutely sure) and no other mods in the packages folder, game is up to date (checked with the program for it, yes) and still I get the error. So it pretty much has to be a custom content issue. I'm just so rusty in these things not having played in a long time that it didn't occur to me.
Thanks for all the valuable help!
Edit: Don't know for sure what did the trick but at least I got it working, might have been a botched patch in the end. I made a fresh start installing and patching everything. I've added the store stuff back but decided to make a fresh start otherwise, so I didn't even put my old save games back. I was sick of those old jackasses anyway, that's why I stopped playing so long ago.