Solution : https://service.sap.com/sap/support/notes/331007 (SAP Service marketplace login required)
Key words : 
additional key words cn21, conversion exit conversion_exit_matn1_input processes, previously pressing 'enter', attached program correction, program error occurs, shortened material number, material number entered, incorrect material number, 'storage location' field, material number
Related Notes : 
       
| 386419 | Confirmation: Input help for storage location -> dump | 
| 132949 | Conversion exit MATN1 |