Unfortunately I think this is a third party issue. The OJS OAI interface states that it’s compliant with the schema http://www.openarchives.org/OAI/2.0/oai_dc.xsd. That file refers in turn to http://dublincore.org/schemas/xmls/simpledc20021212.xsd, which looks like it now redirects to https://dublincore.org/schemas/xmls/simpledc20021212.xsd (note the https). It looks like the validation tool used by BASE can’t handle the redirect to https and is failing. This doesn’t mean that your OAI interface can’t function, though. Short of ceasing the use of the http://www.openarchives.org/OAI/2.0/oai_dc.xsd URL entirely (which I think would be a bad outcome), there’s nothing we can do within OJS to change it.
Regards,
Alec Smecher
Public Knowledge Project Team