Hi Jörg, you're running into a bug there that happens with Offset.

The unwanted grid snap behavior happens because Offset actually calculates a totally new plane for the mouse to snap on to which it derives from the object. That's so that you can do offsets on any kind of planar object that might be at some different angle to the actual view's drawing plane.

Like for example if you draw a circle and then rotate it around in different views so that it is not aligned with the view orientations you can then still use Offset on it and the mouse for the offset pick will track in that circle's plane, not actually in the view plane.

So anyway to support that type of offset where it works on any kind of planar object, the offset command derives a snap plane from the input curve and the origin of that plane (where the grid snap will actually be based off of) will be at the centroid of that curve instead of at the construction plane origin.

I've added that to my bug list to fix up in the future - it should be possible for me to look for the case where the curve's plane is parallel to one of the view planes and then use the construction plane origin for those cases.

In the meantime to solve your problem you can instead draw a single point snapped on to your desired target grid location (It's under Draw curve > More > Pt), and then snap on to that point with an object snap inside the offset command and it should give you your desired result.

Let me know if you still have any difficulty getting the result you need with the alternate method.

- Michael