Thanks for your message. It was the URL used in the relationship (it should be http, not https). The element should instead look like this:
<program xmlns="http://0-www-crossref-org.libus.csd.mu.edu/relations.xsd">
<related_item>
<description>Earlier version of a database</description>
<intra_work_relation relationship-type="isVersionOf" identifier-type="doi"
>10.5479/si.GVP.VOTW5-2022.5.0</intra_work_relation>
</related_item>
</program>
Attaching the revised XML, which is parsing for me now.