I wonder if deleting a LastPass account with the devtools opened would have showed us the full error message going over the network. Or maybe the server is truly just sending back "A".
I contacted LastPass support twice over the phone about the suspicious login attempts. I was shrugged off twice.
I contacted them again by email yesterday, adding the link to this thread, and mentioning the number of similar ip addresses that all attempted to login, presumably with the knowledge of the master password (or a workaround around it? or the password's hashes?). I'm hoping it gets escalated.
Also, I have not contacted LastPass about the error that shows up when deleting an account. I'm keeping my account around since I want to have access to the entry logs related to all of this for now.
See: https://news.ycombinator.com/item?id=29708961
One commenter noted that it wasn't possible to login again (after that error happened), so presumably the account was deleted...?