Solved by myself.
I changed the language code of the keyboard layout to: de453 in the DOSBOX config-file. Seems to be, auto-detection doesn't work on Amiga DOSBOX. So far, so good ... but ...
... there is still another issue and I don't have any idea how to fix it:
Keyboard layout is OK now and the cursor is blinking and when I touch specific letters on the keyboard, for example e , DOSBOX hangs = no more cursor blinking, can't run the keymapper with CTRL-F1 and so on. I have to reset the virtual Amiga (WinUAE 4.2).
Same behavior with a fresh downloaded DOSBOX.
Anyone any idea ?