Hi Pilou, yes that is a bug in the filleter where it is getting confused about how to trace the fillet rails in that situation when the sphere's pole is in that particular alignment.

I was able to fix this up for the next beta release though, so that your posted example will work properly in the next update.

I've also made some adjustments so that the filleter will try not to fail entirely if there was a problem with one particular area.

Thanks for posting the example file!

- Michael