sorry for the confusion but I finally found the reason for my problem.
I set the variable "org-time-stamp-rounding-minutes" in my .emacs correctly but unfortunately I didn't recognize the same variable in "custom-set-variables" section. :(
@Bastien: In the meantime I installed the actual version "Org-mode version 6.28e"
Thanks a lot for all your efforts.
Kind regards, Frank
PS: At least I learned some aspects of lisp debugging. :)