I've got the same problems. The bug is located in the method
org.openxml4j.opc.internal.marshallers.ZipPartMarshaller.marshallRelationshipPart(PackageRelationshipCollection rels, PackagePartName relPartName, ZipOutputStream zos):

targetValue = (new StringBuilder()).append(uri.getScheme()).append("://").append(uri.getPath()).toString();

On the one hand I think somthing like uri.getHost() and uri.getPort() is missing.
On the other hand this implementation would not work with the PDF file example above or URIs like "mailto:someone@anywhere.org"