Periodic 1-way IP network outages

What I have:
Fresh, minimal (mostly default) Ubuntu 20.04 installation w/ UrBackup server from ppa:uroni/urbackup.

What I see:
The server will suddenly start ignoring all IP traffic from it. (Active TCP connections will hang, no new incoming network traffic – TCP or ICMP – is acknowledged). This will go on for maybe 10-20 seconds (varies), then it’ll be good for an average of half a minute. Overall, I’m losing 35-40% of my pings when tested over a 45min period.

When it does NOT happen:
Stopping the urbackup service solves the problem (mostly; I did see it drop out for ~15sec once in an hour); starting the service brings it back.
I sent a bunch of UDP broadcast packets from the server and verified that it didn’t cause any problem.

How to “fix” it when it’s happening:
Initiating any kind of IP traffic from the server will instantly stop the problem (rather than having to wait 10-20 seconds). A ping or an outgoing TCP connection will work equally well.
I can even fix it by using “arping” from another device; the lower-level Ethernet packet is NOT subject to the problem and will be processed, and the returned “I have that address!” response will get traffic flowing again.

What I haven’t done yet:

  • installing & running a packet capture on that machine (I’ve wiresharked it from my PC). I’m not sure that it would tell me much besides either yes/no on whether the incoming-but-ignored packets are being seen at that point.
  • install a 2nd NIC and see if it has the problem.

Any other ideas?
This is a pretty crazy problem!
It’s a default install, so I don’t think I need to go looking for some IPS w/ a really whacky rule or anything like that. I can’t think of what URBackup might be doing that’s unique. I also don’t see anything popping up in “top”.

A different NIC had the same problem. I ended up wiping it and installing the commercial appliance image, which does not have the same problem. It does actually exhibit the behavior occasionally, but overall it’s less than 0.5% loss, a vast improvement. So, I never got to the bottom of the mystery (and won’t, at this point), but at least this provides a little more info should someone else ever run into the same problem.