Hello all,
with the great help of Sumit Bose from sssd- mailinglist the issue is solved:
"ah, can you try to add 'use_fully_qualified_names = True' to the [domain/...] section of sssd.conf, restart SSSD and try again? Now 'wbinfo -s ....' (lower-case s) should return a result as well."
That did the trick!!
Stefan