Yes, so I'm playing on Steam, and I suspect the whole Steam and Ubisoft account linking, and the fact I'm playing on multiple accounts, combined with bugs, might be causing the problem.
I still managed to do the Summit on 4 accounts last week, and have done it on 3 accounts this week. But it seems completely random whether it runs up without crashing or not. Most of the time it crashes, but sometimes I manage to get it to run up, but if I try it again doing the thing I tried before it worked, it doesn't work.
As an example of a bug - if I am on account A on Steam, but Ubisoft is logged in with account B, rather than the one linked to A, sometimes it refuses to run up, flagging an account problem, but other times it just runs up, and when it does that, it copies all the achievements across between the two accounts!