What firmware versions? I feel like there’s some sort of bug with UDP in recent firmware versions - in my case, it’s on my Balance One, and the bug manifests as a failure to connect between a LAN and a VLAN - with ICMP messages showing “Destination unreachable: no route to port” See Outbound Policy + Firewall + VLAN bug in firmware 8.5 - #17 by soylentgreen
In my case, the bug comes and goes - with latest 8.5.2 betas, it ran for 10 days before starting to fail again.
The fact that you are having trouble with
(A) UDP
(B) Firewall
(C) it’s intermittent
… makes me wonder if these are symptoms of the same underlying issue?