OpenVPN suddenly working one way only

Hello.

Not sure this is a question for FreeBSD or for OpenVPN directly... I'll 
try here first.

I'm using OpenVPN quite heavily, as I have around 10 server-server 
tunnels, and several server-clients installations.
They are all working properly except one, which will periodically start 
misbehaving.
Both ends are FreeBSD 13.3, the protocol is UDP and I'm using tun 
interfaces.

Simply put: handshake is gine, packets from host A to B get through, but 
packets from B to A do not.
I can run tcpdump on both tun interfaces:
_ if I ping A -> B, A sees packets going out, but none coming in, 
although B sees both;
_ if I ping B -> A, B sees packets going out, but A sees nothing.

Restarting openvpn on both ends does not help: handskake happens again, 
but the situation does not change. Looks more like a kernel/tun problem...
Possibly rebooting (A or B?) would solve, but I can't do that easily.
Also, I'm sure in some days (possibly weeks) it'll start working fine 
again by itself (!!!).

Notice that both ends have other OpenVPN tunnels to different systems 
and they keep working while this one is not.

Has anyone else seen something similar?
Anything to try/check now that I'm getting the problem and I have no 
urge to solve?

  bye & Thanks
	av.