Its always been bit hit and miss for me. Currently login and navigation takes about 20 seconds each button press for me.
Optus recently upgraded the MYACCOUNT pages but it still seems to be a WIP - I can log in to mine but its essentially empty (No profile data, plan info, etc.) Now its just logged me out unilaterally. Just glad not to be relying on it frankly.
You can try another browser perhaps?
@ss_t - What type of error message are you receiving? As Peter suggested, try another browser as well as clearing your cache & seeing how you go.