Veuillez m’excuser de parler en anglais:
I have come across this issue and managed to solve it by adding an iptables rule:
iptables -A INPUT -i eth0 -p udp --sport 1900 -j ACCEPT
Note that it is the source port which flags the packets which are relevant, the destination port of 1900 is already open (at least in my case). (My solution may be more general to omit the reference to eth0, I have not tested that).
Following the discussion in the yunohost github issue #620 (note that the URL has changed compared to when same issue was referred to earlier in the thread, but I cannot include a link here, for some reason), I created a file in
/etc/yunohost/hooks.d/post_iptable_rules/001-fix-upnp
with the above iptables script. Having done this, UPNP port forwarding works fine if I do:
sudo yunohost firewall reload
sudo yunohost firewall upnp enable
(I am running on a Raspberry Pi 3, behind a consumer ISP router)
regards
David