Posted a bug to launchpad
https://bugs.launchpad.net/inkscape/+bug/1567661

After applying the effect to 2 paths, it crashes when I try to add it to a 3rd...

Just wondering if anyone else has had this problem. It's a super-useful effect so it would
be great to get it working better (if it's not just me that's having the issue).

Posted an Update on the bug page. I think it was the path in question that was the culprit.

This workaround seems to help things (?)
Select path>Break Apart>Union>Apply Fillet Chamfer effect to path

Anyone else notice this behaviour?