Hmm, this sounds vaguely familiar but I thought it had been fixed (or at least worked around). But yeah - enabling ipv6 support will fix it.
(You might want to put “ipv6” in /etc/modules so it is loaded on reboot)
edit: ah, right, here it is: ads-b-flight-tracking-f21/problem-with-piaware-3-1-0-t38122.html
It’s a bug somewhere in tcl itself that I haven’t been able to track down yet, triggered by having an ipv6 address for localhost but not having ipv6 socket support.
You can work around it either by enabling ipv6 (as you did) or by removing the ipv6 address from /etc/hosts