Trying to run doc.serialize(schema="XRECHNUNG"), I get the following error:

"Element '{urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100}CrossIndustryInvoice': No matching global declaration available for the validation root."

While doc.serialize(schema="FACTUR-X_EXTENDED") works completely fine.

Is there anything specific I need to do to make this work?

Please let me know if you need a more complete example to reproduce this.