Jitsi stops when third party enters the room (not behind NAT)

When I run a jitsi call with two people, everything works fine but audio and video shut down when a third person enters the room.
I am running on Debian 10 and followed the official instructions. I had the same issue running on Ubuntu… (it does make it look like I’m doing something wrong)
I am not running behind a NAT, but on a virtual server (8 GB RAM, 4 cores, 100 Mbit/s).
Any help is welcome!
Edit: All required ports are open… I’ve checked according to the official docs.

did you check with this procedure:

(server)
sudo systemctl stop jitsi-videobridge2
nc -l 10000 -u
(workstation)
echo "123" | nc -u (your public address) 10000

No, I haven’t used netcat. But I have used ufw with the following result:

ufw status verbose
Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing), deny (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
80/tcp                     ALLOW IN    Anywhere
443/tcp                    ALLOW IN    Anywhere
10000/udp                  ALLOW IN    Anywhere
22/tcp                     ALLOW IN    Anywhere
3478/udp                   ALLOW IN    Anywhere
5349/tcp                   ALLOW IN    Anywhere
4443/tcp                   ALLOW IN    Anywhere
10000:20000/udp            ALLOW IN    Anywhere
80/tcp (v6)                ALLOW IN    Anywhere (v6)
443/tcp (v6)               ALLOW IN    Anywhere (v6)
10000/udp (v6)             ALLOW IN    Anywhere (v6)
22/tcp (v6)                ALLOW IN    Anywhere (v6)
3478/udp (v6)              ALLOW IN    Anywhere (v6)
5349/tcp (v6)              ALLOW IN    Anywhere (v6)
4443/tcp (v6)              ALLOW IN    Anywhere (v6)
10000:20000/udp (v6)       ALLOW IN    Anywhere (v6)

checking ufw is just asking the OS to open the port, it does not check is the port is actually working. Netcat is doing that.

Right. That makes sense. This is what happened:
I ran nc -l -p 10000 -u on the server.
When I ran echo "123" | nc -u (your public address) 10000 on my home PC, the string 123 was displayed in the command line of the server.
I assume now that the port is indeed open. (Thanks, I learned something I didn’t know this morning!)

Right, then next obvious problem: Debian and java. You installed adoptopenjdk right ? if yes, uninstall it and get java 11 from standard OS repo.

Correct. I went through the trouble to install version 8 as described in the official docs…
I have removed that and installed openjdk-11-jdk as per your recommendation now. (It wasn’t available in the standard OS repo, though.)
Edit: I meant to say I installed openjdk-11-jdk, not adoptopenjdk-11-jdk as I first mentioned.

it’s not java 8 that is the problem, it’s adoptopenjdk itself. AFAIK running apt install default-jdk will install Java 11 from Debian repo.

done that (I ran apt install default-jdk and Java 11 was installed)

Edit: oh and by the way: now it works!
Thank you @gpatel-fr for taking the time to help a novice. This will help my family to meet for the holidays!