[Brian in Charlotte]

MAYBE made some headway here...I truly hope this helps someone. After doing much reading etc., I finally found something on Sweetwater that may have helped:

"In Pro Tools 9 and up, try changing your Playback Engine. This can be done from the Setup menu > Playback Engine. If the Assertion error is preventing Pro Tools from opening, try holding the “N” key on your keyboard while Pro Tools boots up. This will force the Playback Engine dialogue to open before Pro Tools initializes the engine. If the Assertion error disappears, your last session was likely corrupted. "

I tried this and when PT booted up (while I held "N") the dialogue box appeared and sure enough the Playback Engine was on ProTools Aggregate and not my Focusrite. I changed it to my interface and the session opened fine. I just imported session data from the bad try and I'm back. WHEW