"You have entered an invalid username or password. Please enter the correct details and try again. Don't forget that the password is case sensitive. Forgotten your password? Click here! You have used 1 out of 5 login attempts. After all 5 have been used, you will be unable to login for 15 minutes."
Has this issue continued if you attempt this on a different browser/ app on the Mac? If you can log in on one browser and not another, that may have to do with the browser's setting or system updates. If you can't find anything from here, and are still only able to login from a PC and not a Mac, let me know what version of Mac and Safari are being used and I can pass this to the tech team to see if they can help resolve this with you.