That's possibly the issue then (although you say its been working fine before this?).
Optus Sport is only available in Austalia and a VPN can make it appear like you are outside the country. Perhaps the place you are in right now has some sort of external VPN ID?
Others might have some suggestions but I can only suggest contacting Optus LiveChat / Facebook and getting tech support. They can likely see your login attempts and tell you why they have been refused.