Hello Wilbert,
What is EHP version of your ECC system? If EHP5 and above then please check you have created SAP_PROXY_ESR RFC destination, also check for correct credentials are maintained in the RFC destination and also hope you have maintained correct entries in SLDAPICUST t-code.
If below EHP5 then you need to create SAPSLDAPI and LCRSAPRFC RFC destination.
Regards,
Nitin