There are some flaws in the DIM2DataCite crosswalks which is used to register DOIs. The current crosswalk uses dc.date without qualifier which DataCite doesn't except and ignores the maximum occurence of some fields. While fixing the crosswalk I updated it to use the DataCite Schema version 3.1. There were no big changes between the schema versions. The biggest advantage of 3.1 out of my point of view is that we do not have to ensure the order of the different elements anymore.