I don't have access to your panel so I can't say whats going on with your account.
However while mailboxes are not domain specific, e-mail addresses are - and the new panel seems to require that each mailbox to have at least one e-mail address, lets call it a "primary address". "secondary addresses" forward to the "primary address".
For example on my account, I had a user mailbox, and domainA and domainB. Originally I had user@domainA as well as alias1@domainA, alias2@ domainA each deliver to that user mailbox. Then on domainB I had postmaster@domainB deliver to that mailbox. Before the changes were made, I removed the user@domainA address.
When the change was made, postmaster@domainB become the "primary address" for the user mailbox and all the other addresses were set to "forward" to that address. If I want to add user@domainA back, I have to set it to forward to postmaster@domainB because its the "primary address" for that mailbox.
Perhaps your problem is that you don't have a "primary address" for your mailboxes - I don't see how one is supposed to add an address to a mailbox that doesn't have one, and I don't know how they went about determining the "primary address" on my account, maybe on yours they didn't do it at all. Either DH has not completed the changes or they did not think this through.
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