Hello,
UPDATE: strict support was added in phpdocx 10 (Premium licenses).
The problem is that the DOCX uses the strict variant (w:conformance="strict"), that doesn't use the same XML namespaces than the transitional variant.
Strict variant support is a work in progress (its support is beta and it's being tested), and it will be included in the next release of phpdocx. You can use the transitional variant resaving the DOCX.
Regards.