Thank you all, for the helpful and rapid responses.
Of course it was a configuration issue. Starting from square one with OpenLDAP 2 weeks ago I had not grasped the concept of multiple database definitions and not gotten the picture with moving the overlay syncprov into the database section.
Now it works perfect :) and the man pages have once again proven they are worth spending (more) time on...
Please do accept my apologies for not getting this on my own and polluting the list, but after considerable time spent on "goose chasing" I had try other ways of solving the problem.
Sincerely Andreas Taschner