Solution : https://service.sap.com/sap/support/notes/509529 (SAP Service marketplace login required)
Key words : 
terms db2 mvs os390 zos failover reconnect st04 reason, collective ccms transport relevant, sap failover mechanism, call transactions db2, ccms service level, connected db2 subsystem, data sharing group, prerequisites - solution provided, ifi data collector, 'switch database host
Related Notes : 
       
| 426863 | DB2-z/OS: DB Performance Monitor/IFI Data Collector | 
| 324739 | DB2 z/OS: CCMS transport for 4.6D | 
| 217093 | DB2 z/OS: CCMS transports for 4.6C | 
| 191215 | DB2/390: Transports for 4.6B |