Hi,
At the end we contacted Microsoft Support and after a lot of tracing we found there were orphaned records in the invoiceextensionbase table that did not had a corresponding record in the invoicebasetable.
But why this happened we could not say; after cleaning the orphaned records, the import succeeded. I can only give you the advice to check whether you have orphaned records in some base/extension table that do not correspond to a record in it's extension/base.
Good luck.