Having read the other posts about this error, I am creating a new post to get individual help on this.
The pinned post at the top of the email forum from 18 July 20 says that the issue is being looked into and gave a reference number F008135072. Wonder if there is any progress on this? The issue started for me, as with others, when the account was locked due to suspicious activity and autoforwarding being set up, I then changed the password and after getting the webmail unavailable message, this changed to the 'Forbidden' error. Mail is working fine on other devices, so it's only webmail is not accessible.