Check to see if within the AC3 tag of your songs have any weird characters…like weird symbols, writing in forign language, etc.
Traktor can only read characters that are normal symbols characters or numbers. If any of your tracks have anything written in the AC3 tag that is in an asian language or really weird blocks or anything like that then traktor will not read it. When you convert to an ACC copy im pretty sure that it will remake the song but does not necessarily copy all the tags and that is why the ACC version works.
This may not be the solution but check it out….it was driving me mad and took me like a week to figure out wtf was happening.