So, I took a character, changed the name and id of them, and then was just trying to make them appear as either mixed or hybrid. I was specifically changing the 'race' and 'race_display' to 'Human' and 'Fox' respectively. I also changed higher up (but in the section for the same character) the values in 'genealogy' of human and fox to 76/24 respectively, just changed them to 65/35 and it still didn't work. Character comes up blank.
It seems that if 'race' and 'race_display' aren't the exact same, then the character page blanks itself. If they're the same, but an invalid character type (Halfkin Dragon for example) then it loads fine but clicking on the highlighted name thats invalid forces me to close, which is expected.
EDIT: My error logs show the exact same errors as Fibian (a few posts above mine) except for the 'slave_tab.gd' number (which isn't surprising) so I'm decently sure his problem has to do with this.