FCX-03: The embedded invoice attachment is not named 'factur-x.xml' as the…
What it means
The embedded invoice attachment is not named 'factur-x.xml' as the current Factur-X specification requires (some legacy ZUGFeRD producers instead use 'zugferd-invoice.xml', which is accepted but deprecated).
How to fix it
Rename the embedded attachment to 'factur-x.xml' in both the Filespec (/F, /UF) and the EmbeddedFiles name tree.