can't keep connected more than 30 seconds with default server

Hello,

I try to set up a Minecraft server 1.14.3 on Ubuntu 18.04.1 on my pc. I've already set up a server some years ago and it's the same method, which is:

• launch server
• accept EULA
• relaunch server
• open port 25565 and redirect router to server

So I try with defaut server, default options, nothing special (no whitelist, default port ...)

I can connect to the server using local ip, but others players can't keep connected more than 30 seconds. Server prints

[13:13:51] [Server thread/INFO]: *** joined the game
[13:14:07] [Server thread/INFO]: *** lost connection: Timed out
[13:14:07] [Server thread/INFO]: *** left the game

Client works well on others servers, and the other player tried to set up a server and it's the same problem.

We don't use mods or modified clients (default client also)

I activated allow-flight, pass "java -Xms1G -Xmx1G -jar server.jar nogui", op the player,  and the problem disappears. I tried with a new fresh server but I can't say what solves this problem.

So I think there is a bug of the default server configuration

If you need others explanations or an access to my fresh server, don't hesitate.

Best regards,

minepiou