Hi Mark
Please cross check input values while executing BAPI_INSPECTIONPLAN_CREATEin task table for below components.
TASK_MEASURE_UNIT
TASK_MEASURE_UNIT_ISO
and also cross check the material BUOM with above component inputs.
If all are correct then check ALE/EDI settings for this UOM in CUNI.
Regards
Chennakeshava Rao