I can't find any mention of this around, which means it might be a side effect of the trainer. Ppl who had this error, please write here your level, the state of your skills (all maxed or normal) and what options you were using while this happened.
This error could be happening because you have a quest item that gets glitched by the add inventory items trainer option, in this case most likely the "Codex of Destiny". If so, your only option right now is to load an earlier not glitched save or somehow hex edit your save file to make the 10 "Codex of Destiny" item into 1 again, since quest items aren't meant to stack on each other or have more than 1 of each, and you can't sell it or add it to junk to destroy since it's an quest item.
People with the glitched quests could ask Caliber or PWizard to add a "reduce inventory items to 1" option to the trainer, as this would be the easiest option if you don't have an unglitched earlier save. If wasn't the case then waiting for a patch would be the only option.
Weird, I have the xbox 360 version also, but I didn't get bug at this part in the plot, I was able to advance the plot line a lot farther, before I decided to give the PC version a try. My PC version is patched up to the latest patch though. The only thing I can think of is that the recent patch must of broken the game plot somehow, as my xbox 360 isn't connected to the internet and the game works just find, haven't noticed any bugs or glitches other than the damn bad/blocked camera angles.
My bug i ran into began with the faction quest line that starts at the House of Ballads. Furthing down into this quest line you come upon the quest called "the champions" I'm suppose to loot the armor to get into another area but the guy i'm supposed to loot the armor off of isn't fighting me even if I force it with hitting "H". He just takes the hit like a biatch and doesn't fight back. Fuuuuu buggy games like this makes me so irritated. Oh well. Back to Final Fantasy XIII-2 and wait for Tera to be released.
Happened to me too...same part...go talk to Agarth at Three Lamps Inn...no options to move forward after he asks, "You ready?"
Pretty sure that the trainer broke that quest somehow...got friends farther than i am, no trainer used, and they didn't get stuck on this part. This is pretty crappy to be honest...i didn't even realize it was glitched until i spent the next 8 hours doing sidequests...and when I went back to do the main quest....bam. Yikes.