That's exactly the error I had too, forcing me to go back to an earlier save. I've found out how to avoid the error, and how to reproduce it.
It's related to a problem that someone else noticed, where you can't get Cassie to follow you to the second camp. When Olivia discovers the cliffside camp, she says that she would like to take Cassie there. Now if I immedeately do a fast travel back to the first camp, I can talk to Cassie normally and make her follow Olivia. When I take my time though, have Olivia sleep for a night (she's getting horny as usual), and then walk back by foot to the beach camp, the only possible interaction with Cassie is having sex with her. After they're finished, Cassie completely disappears, and I have to wait until next day for her to be back, again with only the sex option available.
The fatal error triggers once you control Cassie, and she arrives at the entrance to the cliffside camp for the first time. The game assumes that she's with Olivia, but actually she isn't.