What is forwarding with masking?
It only causes the error with the masking feature. I've searched but can't find a solution or what would cause it.
https://www.godaddy.com/help/whats-the-difference-between-domain-forwarding-and-masking-32147
What they're calling forwarding with masking is not forwarding at all. They probably just use a frame to display the actual website. It doesn't work because of the cookies restrictions on frames in modern browsers. You shouldn't use it. Just configure domain properly.
The error doesn't happen when I log into the modern interface.
When I use a 301 or 302 redirect, I have no problem logging in.
The error I get is:
And thanks for the great server!