Hi, some years later, i have the same problem:
The schema(s) used for schema-validation had no errors
709 schema-validity problems were found in the target
Schema resources involvedAttempt to load a schema document from
[external links are visible to admins only]
(source: schemaLoc) for
[external links are visible to admins only],
succeeded
Problems with the schema-validity of the target
[external links are visible to admins only]: Invalid per cvc-complex-type.1.2.4:
element {http://www.sitemaps.org/schemas/sitemap/0.9}:lastmod not allowed here (5) in element {http://www.sitemaps.org/schemas/sitemap/0.9}:url, expecting [{Wildcard: ##other, strict},$]:
[external links are visible to admins only]: Invalid per cvc-complex-type.1.2.4:
element {http://www.sitemaps.org/schemas/sitemap/0.9}:lastmod not allowed here (5) in element {http://www.sitemaps.org/schemas/sitemap/0.9}:url, expecting [{Wildcard: ##other, strict},$]:
[external links are visible to admins only]: Invalid per cvc-complex-type.1.2.4:
element {http://www.sitemaps.org/schemas/sitemap/0.9}:lastmod not allowed here (5) in element {http://www.sitemaps.org/schemas/sitemap/0.9}:url, expecting [{Wildcard: ##other, strict},$]: