I tried to change password after first boot with cli. I have used the command passwd and this affects the root login. Password was changed.
Before Version 2.7.x that has also affected the admin password für Webui login. But in 2.7.x the admin password still stays on the default.
I have also used this command to prevent first webui login asking to change the password: uci delete vuci.main.firstlogin='1'
That was working. The only thing what does not work is setting the admin password for webui to the same password as root login at cli.
Thanks a lot for help.