Mail has been very bad for many many weeks.
I doubt you have been hacked. I've had a similar "accessed by another device" type error on one day about a week or so ago.
When I had that error it only affected accessing via via a mail program (using POP3), but I was able to access it via web mail, even though it was slow.
The error message comes up when I try to download email messages on Outlook.
Mine was with outlook as well.
When I try to login into Webmail I get error message "wrong username or password".
I've have that once this evening, but I was already logged in. A couple of refreshed and I was back in.
Whatever you do don't be tempted to change passwords unless you know for 100% that the password has somehow changed (or has definitely been compromised), otherwise you are likely to get into support hell loop with Optus.
Looks like it is the Optus Server issue rather than anything else but cannot be sure till we can get in contact with Optus Support. Not going to be easy to get hold of them to get a valid answer.