Finally, I've found an error in my situation: The .hdbprocedure was stated within the wrong schema:
Changing:
PROCEDURE "DEV_xxx".
into
PROCEDURE "NEO_xxx".
solved the problem.
(Be sure to call the "HCP"."HCP_GRANT_SELECT_ON_ACTIVATED_OBJECTS" and the "HCP"."HCP_GRANT_ROLE_TO_USER"('<model_access file>', '<user name>'); after activating.