Have you tried creating the LSMW script with transaction code PP02 instead of the usual OM transactions like PO13, PO10 or PO03? We've generally had most success recording LSMW scripts via PP02.
Since you mention IT1000 specifically, it may be how you're handling the number range assignment, if you're creating new OM objects. For example, if your system only allows internal number range and you're specifying a object ID, or vice versa.