Worked a week ago, now can no longer find other devices

[ReaderGuy42]

I'm on Linux Mint 20.1, don't know what Android/App Version, but it should be the most current because there's no available update in the app store.

A week or so ago I got it to work, so I could send files back and forth between my phone and my PC and it worked fine (though I weirdly had to change the group code to something else, which made it work, even though they started off with the same default name, with which it hadn't worked)

Now I tried doing the same thing today, with nothing changed (unless there was some update I don't know about) and both the app and the Linux program can't find the other device. However I have the feeling that the app isn't even looking, meaning it tells me so fast that there's no other device, I'm wondering if there's a connection issue?
They're on the same wifi network and so on, and again I haven't changed anything else.
I also updated the Linux firewall rules just to be safe, but that didn't do anything either.

Any ideas?
Thanks!

[slowscript]

Well, there was an update to this app about 6 days before your comment. I don't think it changed much in terms of device discovery though. You can test it with the old one by downloading it from the Releases page.

This app unlike the Linux version doesn't show any indicator that it is searching. "No devices found" means that no devices have been found yet. It starts looking at the moment it's service starts (the red "Service not running" bar disappears) until you close it. So unless you get any error popup or something strange appears in the logs it should be looking all the time. Sometimes it takes 5 sec to find the other devices, sometimes it takes a minute or two. Sometimes it helps to restart it (tap "Quit" in the menu).

I also don't think you had to change the group code. It plays no role in discovery. What you described is likely nothing more than a coincidence.

I've now had a bunch of reports that device discovery is not reliable so I'm wondering what could be the issue. The way it works is that it sends 3 UDP packets in rapid succession announcing itself and requesting other devices to do the same. If these packets get lost for whatever reason, you have to wait ~2 minutes before the process repeats. Is it possible that weak WiFi signal could cause this? If you have a laptop or a PC with WiFi, could you please test if it gets any better when using your phone's hotspot? This way you have a network with only two devices that are close to each other. If this turns out to be the case I'll probably add an option to do the announcement manually or at least increase the frequency it happens.

Using Linux Mint 20.2 with Warpinator 1.2.4 and the Android version.

Not sure if this is connected, but in the Warpinator preferences on Linux you are able to 'Update firewall rules' - the issue is it only updated TCP rules for me. Which still resulted in certificate errors and requests to update my firewall rules on my Android device.
I added UDP rules manually (the selected blue ones in the screenshot) to allow incoming at 42000 and 42001. Then it worked.

[Lee-the-Geek]

I can only connect devices to my Linux Mint machine after I disable ufw. Warpinator is allowed in, but my external devices are not found.
As soon as I disable ufw, all external devices become available.
At least it's a work-around.