Solution : https://service.sap.com/sap/support/notes/1369901 (SAP Service marketplace login required)
Key words : 
system raises error message bm305, apply relevant support package, advanced program correction, symptom transaction kepm, case-sensitive entry, error bm305, case-sensitive, measure values, language y', input check
Related Notes : 
       
| 1224961 | KE21N: Unit of mesaure in lowercase causes error BM 305 | 
| 789748 | No lowercase letters for unit of measurement IDs | 
| 619937 | Units of measurement and conversion exits |