Sebastien,
Adding the Fonts to the Windows Server is NOT sufficient you need to update the XML file so the BI Platform is aware it. Please see additional information to couple of SAP notes that tell you how to add the fonts to the Platform. When you schedule the report it uses the Platform, this is why you need to update the XML file so the internal servers are aware if it.
http://service.sap.com/sap/support/notes/1907396
http://service.sap.com/sap/support/notes/2010036
Regards,
Ajay