soundcloud.com dos not accept mailbox.org

Tried to change my Soundcloud account email adres from gmail to mailbox.org.

It returned "An error occurred while saving. Try again.". So i tried all small letters for my alias and main mailbox.org account. So i tried Microsoft "@live.nl." And that did register. But thats not wat i wanted.

All companies are american and not GPDR, i would still like to change to Mailbox.org (Germany). I could not find any reference on "Soundcloud + Mailbox.org" on google. Has anyone encounterd this issue too?