"404 Invalid username or password" error when using multi-session (multiple accounts)

When the message "404 Invalid username or password" appears while using multi-session (one person with multiple associated accounts): 1) Ask them to log out of all accounts and log in again. 2) Check that the email address or password does not have spaces at the beginning or end. 3) Have them update the app. (Multi-session error; this is resolved by logging out and logging back in.)