So this is all you did?
For your class name and description, don’t make those 0xFFFF. Put those to real name values just like in the character editor. I bet that’s what broke it.
Your max HP probably shouldn’t be less than 60, but I don’t think that should break stuff in that way. I’ve just never seen that done.
Otherwise, it looks okay.
(By the way, when posting large images like that, use spoilers in the extra post settings. They can hide images and walls of text to make it easier to read. )
Is it intentional that the class name/desc use 0xffff? With the evidence you’ve given, it seems like the game crashes whenever it tries to display the name of the class (statscreen and level-up screen). Try changing those to use a text slot with a name and one with a description respectively (the text slots used for dialogue can crash the game if they contain the [A] tag; the game doesn’t know how to handle that when it’s in text that automatically displays in menus).
Also, if you’re using one of the class slots that Nightmare calls “Empty (make a custom class!)”, don’t use those. The game actually does use those for archers getting in/out of ballistae if I recall correctly.