I know this was an old thread, but I looked at the last entry and couldn't see the exact solution so I am going to write it in case anyone like me gets this problem. It might be a good idea to read the whole topic as it contains some helpful tips though. Here it is: As I found it on another forum which was 2 or 3 entries long, the problem is caused by "BECOME KING OF RIVERHELM" mod. Disable it and voila! Problem is solved. For me, at least. I must note this though, as I am writing this now, I haven't searched this topic if people here could find this solution or not, but either way, I thought it would be good for people who would skip reading the topic and see if there is a solution at the end. So yeah, enjoy playing Skyrim without a CTD at "Irileth, send a detachment to Riverwood at once".