Hi,
did it perhaps happen after you switched to the D3D9 graphics client?
I had similar issue with many base meshes. Try to un-check the "Object shadows" tick-box in the "Visual Effects" tab of the Orbiter launchpad menu and see if that helps.
I find that the D3D9 client seems to give shadows to all base meshes when that option is enabled, regardless of the presence or absence of the "SHADOW" flag in the mesh entry in the base config file. That causes edges like that for semitransparent meshes, at least in my case. The correct behaviour (as in the in-line client) should be to show shadows only for the meshes that have the "SHADOW" flag present.