sounds like your missing a line in your Animations file.
tell us a bit more - are you copying the default folder and then overwritting? how old are the ui pieces you're trying to use? pretty much anything in the actual TKing area is going to be dead - it hasn't been updated in way too long.
anything that includes the Inventory window or the loot window has to have been updated since the middle of Feburary (or say TBS compatable). anything that includes the Player window must have been updated since mid-September (or say TSS compatable)
from the amount of time you've been putting in to this based on your post, i'm thinking you're probably willing to spend some more time to get it to work. here is my suggestion.
create a new UI folder called something like Test. put the UI files that you want into it one at a time, and /loadskin in game after each file. if it loads, great, if it breaks, remove the last file you put in and continue down the list. this will take a little bit, but it will tell you which file or files are the problem.
you can put them in 2 or 3 at a time, but it means that you'll have to delete the entire set you just put in when you get your error, and do them one at a time.
btw, when i say one file, you can do the UI piece and whatever images go with it - with the example of a TKing spellbook, you have the UI file (UI_SpellBookWnd or something like that), and 3 or 4 pictures that are neccessary for the inventory, and only for that. put them in all at once.
if you can, you want to avoid having the Animations file in your custom UI. for the most part, it's more of a hassle than it's worth.
Custom UI not taking properly
Linear Mode
