Is there a timeline/history somewhere detailing exactly how AIVAS' final mission was unsuccessful? I'd like to know, if possible, how the events in Triads' history differs from those detailed in The Dragonlovers' Guide to Pern.
If there was an explaination I never heard it. There's over a thousand turns between then and now, so I'd say there is no explaination of how it failed. I think by this point they only have a vague idea of what happened with AIVIS to begin with. They know that in the distant past they had one shot to kill Thread forever, it seemed to have worked, but they had been wrong.
I knew that there was a brief explanation somewhere:
http://www.triadweyrs.org/faq.php?fq_id=490