Hi Andras, if I understand correctly you want to shave off this little slivery piece here:





It can be difficult for the intersection code to deal with cuts that are so thin that they're close to being coincident with an existing boundary.

It would work better if your cut was not so very close to an existing edge - you can get that in your case here by doing an "untrim" to recover the full larger surface which is there under the current trim boundary.

Before you do the untrim you'll want to duplicate the edge curve on the other side so that you can use it during the retrim as well, because when you untrim it, it will extend out past the other side as well.

So select that other opposite edge, and do Copy and then Paste to duplicate it, then select all edges (with one edge selected do Ctrl+A to select all), then push Delete to remove that trim boundary and "untrim" the surface, you'll now have an extended surface and you can now select it and run Trim, using the plane and the copied edge curve as the cutting objects.

I've attached a 3DM model file where I've done those steps except for the final trim so you can see how it looks. Use the plane and the curve as the cutting objects in the Trim.

- Michael