I have tried printing a DHL Parcel UK shipping label and am seeing an error returned:

Validation failed. Items must be less than or equal to 1 


DHL Parcel UK has a restriction for some services to have split packaging applied (more than 1 package for the same consignment).

In order to resolve this error you would need to split the actual order into 2 (or more) parts rather than using the split packaging option. Alternatively you may need to use a different service to ship the order.