Thx now it works fine (so far

)
for all the others with the same prob:
start server first time and get the admin pw.
start client with the user: admin and its pw (from the server.log)
highlight the user (admin) and open the self menu
click on "admin resister player with server"
Set a new admin and pw.
close client
use the server admin tool (web interface ) with the new admin and pw
thats it (at least on my machine)