Hi Serban,
another possible error source is if you use the NetWeaver Business Client 5 with SAP GUI 7.30. NWBC 5 normally only works without a SAP GUI or with SAP GUI 7.40 installed - not with SAP GUI 7.30.
The solution in this case would be to upgrade SAP GUI to 7.40. If this is not possible, please let us know.
Best regards, Sebastian