If an IPv6 host has no AAAA DNS entry the DISPLAY variable might
contain a hexadecimal/colon address.

The function x11_connect_display() in channels.c determins the "hostname"
in the DISPLAY variable by searching for the *first* colon occurence (line 3550 in that file):

    cp = strchr(buf, ':');

This seems to be incompatible with the representation of IPv6 addresses.

The resulting error produces an error message which can be attributed 
to the debug2 statement later in that function (line 3582):

    connect 2001 port 6006: Invalid argument
    Error: Can't open display: localhost:10.0

A workaround is documented here:

   http://www.technologische-hilfe.de/antworten/lightdm-xdm-sitzung-und-ipv6-support-241428392.html

My guess is that the problem can be solved by using strrchr in that statement:

     cp = strrchr(buf, ':');