Solution : https://service.sap.com/sap/support/notes/1404550 (SAP Service marketplace login required)
Key words :
data area r3<sid>400/r3_<nn>, system unexpectedly displays, remote function call, transaction sm51, logon screen, source instance, target instance, sap instance, case-sensitive, default profile
Related Notes :
| 90323 | Display of RFC logon screen in SM51 |