I think that Crossref changed some URLs and apparently those for JATS and AI as well. I’ll take a look i.e. ask them to provide a redirect from old to new URLs, so that XMLs are valid again…
Edit: it was the case also for the main Crossref schema, s. OJS3.0.2 and Crossref Plugin validation error - #10 by bozana and for that they already implemented the redirect…
Thanks!
Bozana