OK. Found the problem. Since Xorg 1.20. Only console users are allowed to run the X server. So... what user does dtlogin use to start the X server? Anyway the temporarily solution is add allowed_users=anybody to /usr/local/etc/X11/Xwrapper.config. After that I can start dtlogin with -daemon argument.
FreeBSD 12.1 dtlogin won't start with "-daemon" arguments after xorg upgrade