Solution : https://service.sap.com/sap/support/notes/209887 (SAP Service marketplace login required)
Key words : 
additional key words v1801, system displays messages, mandatory condition vat, condition screen, 'pricing error, program error, text items, definition generally, subscreen method, branches incorrectly
Related Notes : 
       
| 328240 | Displaying the tax conditions | 
| 326556 | Text items and pricing III | 
| 319356 | Text items and pricing II | 
| 303606 | Billing plan and text items | 
| 206352 | Short dump when leaving the condition screen | 
| 66221 | No pricing for text items |