As I have stated on a post somewhere-I have the same problem- but the cause is very simple----just because two cousins 3x removed married it has effectively "merged" two entire family lines and confused the relationship calculator.
I am no longer straight line related to the generation previous to these two people. The calculator goes to the two marrieds, who were added most recently, and goes up and back down the trees to get to my older ancestor.

Were these programmers A students, I ask?

So I guess I have to un-marry these two people and just make a note that due to ancestry programming, their marriage cannot be shown. Good advertizing as I share my tree with relatives!
I will write to support@ancestry.com.