Quote - Point taken, however have you tried IDL rendering instead of IBL for ambient lighting to see if the same problem occurs.

Yes I have.  The line only appears with IBL and AO.  Every other lighting scheme does not show a line.  If I turn down the AO Scene bias to 0.0004 the line goes away, but so does most of the shadow that I want to retain.  When I render it in Lux through Reality, there is no problem and there is no problem with Firefly's IDL.