We are still investigating with Crossref, but it appears that they may have dropped support for the API endpoint which was used up until OJS 3.1.2.
This same endpoint was used in all OJS 2.x Crossref submissions, and in early OJS 3.x code, including 3.0.2.0.
In an open ticket with Crossref, their support reports that you should still be able to download the XML export from OJS and upload that manually. This should, however, be considered to be a secondary workaround to upgrading to a more recent version of OJS. OJS 3.0.2.0 is extremely old, and is subject to a number of security advisories. [C.f. #3805 #3944/#4024 #5302 #5871]