The bad news is I have no freaking idea how to find out which field it is, which is a very unpleasant situation and one of the reasons why I am about to trash the JAXB approach for ZF1.
The good news is twofold, first that this is one of the rare cases where I accidentally have a very similar problem, so I don't need to ask for further steps to reproduce.
Secondly: it works in ZF2, so if you do sth like
ZUGFeRDExporter ze = new ZUGFeRDExporterFromA1Factory().setProducer("My Application")
already on loading, it should work or at least give you a decent stacktrace.
It may be some attributes in payment for getTradeSettlementPayment.
Still trying to find out.