Hi Sven and Ankur,
Many thanks for your suggestions. I did check the help for vendors and it was correctly working.
I kept looking and finally I found the problem. While the HRP and HRT symbolic account customizing was correct, the posting keys associated to HRP were not. The posting keys were outside the vendor posting keys range. As they did not belong to the vendor range, the system did not identify the account as one of a vendor and then did not try to determine the vendor dinamically by the employee number.
This logically led to the error of No G/L account number *KP . Once the correct posting keys were in place, the system worked as expected.
Anyway, thanks to all for your help.
Daniel