I had the same message as Tacita when I tried to call my guide after returning to the collegium from the ruins, but it finally worked for me a few minutes ago - READ JOURNAL triggered Atropos arriving and the end of the quest.
I had finished the 5 collegium quests but still hadn't graduated, so I waited for the graduation time to tick to zero. Still no Atropos reappearing, even after I read journal.