Unable to change support and newsletter address

I have a self-hosted (Digital Ocean Droplet) installation of Ghost 4, everything regarding the installation went smooth, but somehow I cannot change the e-mail address for support and newsletters. I use a .co domain, and I just get an error message (see picture) when I try to add my own address. But if I change to .com it “works”, meaning the confirmation e-mail is sent but since that’s not my e-mail I have no way of knowing if it arrives in someone’s inbox.

Weird error, any suggestions?