Plex 1.15 on Synology only listens on IPv6

Plex Media Server NAS & Devices
1

Server Version#: 1.15.1.710

Not sure from which version I updated, but I updated to the current PlexPass version on my Synology last night.

Now the PMS Web interface is no longer reachable at the IPv4 address of the DS (i.e. 192.178.1.75:32400/web just gives a connection timeout)

At first I thought it didn’t properly start the web daemon and just restarted both Plex and the DS to no avail.

I know my way around on a linux machine, so I did the usual diagnosis:
Check ps and htop for the Plex processes, everything looks fine.
Then I wanted to find out if for some reason the web port was blocked maybe by the old version still running or some other strange situation.

So I called my good friend netstat to the rescue:
netstat -tulpn

And there I found something strange:

tcp        0      0 127.0.0.1:32600         0.0.0.0:*               LISTEN      5106/Plex Tuner Ser
tcp        0      0 127.0.0.1:32401         0.0.0.0:*               LISTEN      4351/Plex Media Ser
tcp        0      0 127.0.0.1:50006         0.0.0.0:*               LISTEN      4539/Plex Plug-in [
tcp6       0      0 :::32400                :::*                    LISTEN      4351/Plex Media Ser
udp        0      0 172.17.0.1:47974        0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 172.17.0.1:41996        0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 192.168.178.17:36019    0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 0.0.0.0:32410           0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 0.0.0.0:32412           0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 0.0.0.0:32413           0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 0.0.0.0:32414           0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 127.0.0.1:42806         0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 0.0.0.0:1901            0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 127.0.0.1:36902         0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 192.168.178.17:47147    0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 192.168.178.17:43055    0.0.0.0:*                           4351/Plex Media Ser
udp        0      0 172.17.0.1:59908        0.0.0.0:*                           4351/Plex Media Ser

As you will no doubt notice: Plex only listens on tcp6 for port 32400, but not on tcp.
I know, depending on netstat and the kernel version this would be perfectly normal to only show tcp6 and listen on bothe IPv4 and IPv6, but DSM is different here: It still lists IPv4 and IPv6 sockets seperately.

And oh wonder: connecting to the IPv6 address on port 32400 reveals the well-known Plex webinterface.

Why does it do that and how can I set Plex to listen on IPv4 as well? Listening on IPv4 only would be fine as well. I don’t need IPv6 on my LAN yet.