I get below error when I try the Register button for an unregistered article. If I export the xml file and directly upload to CrossRef it works though.
Our server is behind firewall, but I have set http_host and http_port in order to access external network.
Yes, there seems to be something with their deposit API/system and they are working on it. Till it is solved, please export XML from OJS and manual upload it on the Crossref portal. We will let you know once the issue is solve.