It is possible you're doing too many things at once?
For example, if you start adding a relative to Person-X, then it's a good idea to save the relative's new profile before making even the tiniest change to Person-X.
If you get interrupted (like I tend to do) and Person-X is saved again before the new profile is saved for the very first time, then there is a conflict error. The message is really hard to see, it appears in the url for a moment. If that's what might have happened, you just need to start over with Relative A.