Not sure if its been covered before but for information
I found I was getting loops in a imported schedule that I was updating in P6 with a schedule from a contractor. I would only update the activities, relationships, and resources on the activities. All the rest i would set to keep existing. The contractor had made a number of logic changes in the plan. I imported and went to F9 the project after some links to outside milestones and I suddenly had loops appear. I was thinking that they were in the contractors XER they had submitted. I tested thiers as a new project and F9 worked fine.
I removed the loops and imported the plan again and got the loops again. Finally I stopped to read the delete check box in the import modifcation area and I deleted all the relationships in the current plan and replaced the same with the import plan. I didnt have any more loops
Just thought maybe you could use this info for those ever running into the same problem.
I think the import plan was simply adding the logic which was new to the logic already there.
Thank you for sharing this information
Thanks Ron for Sharing this.