Tvheadend

There are actually two bugs here:

1. If you enable the SAT>IP server tvheadend will forcefully start listening only on IPv6, not on both protocols like it does for ports 9981 and 9982:

tcp6 0 0 :::9981 :::* LISTEN 27156/tvheadend
tcp6 0 0 :::9982 :::* LISTEN 27156/tvheadend
tcp6 0 0 2001:xxxx:100:393::9983 :::* LISTEN 27156/tvheadend

2. SAT>IP is still not usable because the URLs described in desc.xml are missing the brackets required for IPv6 URLs:

desc.xml: <presentationURL>http://2001:xxxx:100:393::2:9981&lt;/presentationURL>
tvheadend.log (on client): satip: SAT>IP DVB-C Tuner #1 (2001:xxxx:100:393::2:9983) - RTSP error -104 (Connection reset by peer) [6-0]

When the client tries to scan a mux there is not even a hint of a connection on the server side which suggests that tvheadend does not parse/use the URLs properly.