I went through, unchecked all potentially problematic plug-ins (Massive, Reaktor, Kontakt), also removed the actual VST files from the plug-ins folder, yet the 2 corrupted sessions still didn't load. Both projects hang when loading the MixConsole. One when "Loading Channel: Massive", the other "Loading Channel: BrassKontakt". These are my names for the output channels for the respective plug-ins. At this point, I think the only thing to do is to re-constitute the projects track by track (to add insult to injury, when I try to export MIDI files for the projects, Nuendo crashes). Thanks for the advice!
Nuendo 6.0.6 NEK, Mac Pro 2x2.26 ghz Quad Core Intel, OSX 10.8.5, RME Fireface 800.