Solution : https://service.sap.com/sap/support/notes/556212 (SAP Service marketplace login required)
Key words : 
symptom rfc logon load balancing, rfc logon load balancing, enable rfc load balancing, load distribution reason, numerous rfc connections, application serveroverload due, standard releases below610, 5 minute period resulting, overload ofthis server, numerous rfc logons
Related Notes : 
       
| 605016 | SMLG: short dump SMLG_WRITE_Q_TO_MONI does not exist.. | 
| 593058 | New RFC load balancing procedure | 
| 591792 | RZ20: 0 as 'Standardized Response Time' | 
| 571241 | Load Balancing and the RFC user |