SINGLE EDGES are Expected for a 2D model
 

That Single Edges Check is listed under "possible problems", not Errors.

A Single Edge is just an edge of an element that has no other elements attached to it. That doesn't sound so bad does it?

It could be a problem in a 3D model where you want the exterior shells to be closed before you can generate a delaunay fill because it could represent a hole you need to close... In a 3D model, it could also be little tags of mesh that you want to remove (this is what the auto fix assumes).

It is not usually a problem for a 2D mesh (or shell mesh)... You should expect single edges all around your perimeter!

However, if you see internal single edges where you don't expect them, that may be a problem (a gap or connectivity problem).

Thank you very much! It reassures me! I have indeed single edges all around my perimeter, and only there.
Thanks a lot!