Java Class - error Could not initialize class org.openxmlformats.schemas.drawingml.x2006.main.ThemeDocument

All the other posts aside, I would encourage you that if at all possible that you DO NOT use .xlsx as a transport format. Using POI is okay and all but experience has shown that using Excel formats in this manner is fragile. Relatively easy to break the integration just because a user edited a workbook and to them looks “correct.”

You may find Excel FF to CSV FF comments to be of interest.