I've just had this same problem when trying to get access to my 2FA settings, I'm now getting "Error 403: disallowed_useragent"
So I tried to see if I could access the security settings from the website, but it automatically added this PC as a new device, despite me being at the 2 device limit, then I could not access my notes on the phone.
I have seen in the activity log that someone is trying to access my account, but now because of the error 403, I can't change my password to secure the account.