Solution : https://service.sap.com/sap/support/notes/339043 (SAP Service marketplace login required)
Key words : 
additional key words, fiscal year variant, scheduling screen manually, symptom problems occur, problems occur, scheduling dates, manually change, period 5 manually, demand management, respective period
Related Notes : 
       
| 672255 | Shortened fiscal year/changing fiscal periods | 
| 118600 |