Solution : https://service.sap.com/sap/support/notes/65961 (SAP Service marketplace login required)
Key words : 
additional key words cn21, network activity end, prerequisites program error, latest dates end, indicator 'duration extensible', earliest dates, operation duration, wbs element, project planning, solution repair
Related Notes : 
       
| 120149 | Operation dates in the process order | 
| 60862 | Sub-operation/element: Dates not at working hours | 
| 45759 | Scheduling for exact date with work center |