I just made a trace to have a look. I’ve seen this :
30925 2559.377015 192.168.0.18 255.255.255.255 UDP 60 35623 → 35622 Len=1
30926 2559.388184 192.168.0.186 192.168.0.18 UDP 59 35622 → 35623 Len=17
This is a bit surprising to me because when you send a udp packet to a server, the source port should be random allocated by the IP stack. And when you have a firewall, by default it should “open” the way back. Anyway, I had reactivated the UFW server side, so I have added a rule to allow port 35623/udp and a backup started two minutes later.
I hope everything is now ok.