It seems that we found the problem and Ill try to give an explanation for future similar cases.
It is caused by an abnormal previous termination of BaanERP (ENDBAAN).
At the time of stopping the Baan system, shared memory is not cleared from the AS400 memory. When restarting baan could not load into Shared Memory the appropriated tables (SHMTIME job does not start as it fails to reload the objects into shared memory).
The bw-client couldnt locate those tables (or better table definitions) in shared memory as expected (although they are existing and it could find them directly from disk).
The problem solved after restarting the server (IPL-reboot), but I believe that there must be a better way to reload SHM.
Just for the record you define the shared memory in session ttaad1520m000 (ttaad1120s000) for the data dictionary of a package combination and in session ttaad4150m000 for the objects to be loaded in the shared memory and improves the overall performance of the ERP.