Client: All of Fabric 3 - 2.7.2
Server: All of Fabric 3 Server - 2.7.2

Setup:
All users are using the same client to server combination. Server has port forwarded 25565 with fire-walls open for TCP connection. Same server has been used to hosted paper server and has worked for the past 6 months on the same port. Paper server was not running nor was there any application using the same port during testing.

Bug:
Connection to server from external IP outside of the server's network quickly connects and then disconnects. However, server admins connecting using external or internal IP within the same network is able to connect. I suspect there is another port internally being used by All of Fabric 3 that requires firewall permissions?

Expected Behavior:
Game connects indefinitely or atleast throws an error in server or client logs.

Clients logs do not show any reasonable error logs:

[23:40:32] [main/INFO]: [STDOUT]: New minimap session initialized!
[23:40:32] [main/INFO]: [STDOUT]: New world map session initialized!
[23:40:32] [main/INFO]: Loading slotlock file
[23:40:32] [main/INFO]: Successfully loaded slotlock file
[23:40:32] [main/INFO]: Started 8 worker threads
[23:40:32] [main/INFO]: Environment: authHost='https://authserver.mojang.com', accountsHost='https://api.mojang.com', sessionHost='https://sessionserver.mojang.com', servicesHost='https://api.minecraftservices.com', name='PROD'
[23:40:32] [main/INFO]: Requested config sync for mousewheelie
[23:40:32] [CraftPresence/ERROR]: Asset name "overworld" does not exist, attempting to use an alternative icon "aof_-_tile_cover_test_02"...
[23:40:32] [CraftPresence/INFO]: To add support for this icon, please request for this icon to be added to the default Client ID or add the icon under the following name: "overworld".
[23:40:32] [CraftPresence/INFO]: Fallback icon for "overworld" found! Using a fallback icon with the name "aof_-_tile_cover_test_02"!
[23:40:32] [CraftPresence/ERROR]: Asset name "skk_server" does not exist, attempting to use an alternative icon "aof_-_tile_cover_test_02"...
[23:40:32] [CraftPresence/INFO]: To add support for this icon, please request for this icon to be added to the default Client ID or add the icon under the following name: "skk_server".
[23:40:32] [CraftPresence/INFO]: Fallback icon for "skk_server" found! Using a fallback icon with the name "aof_-_tile_cover_test_02"!
[23:40:33] [main/INFO]: [STDOUT]: Minimap session finalized.
[23:40:33] [main/INFO]: [STDOUT]: World map session finalized.
[23:40:33] [main/INFO]: Stopping worker threads
[23:40:33] [Thread-316/INFO]: [STDOUT]: World map force-cleaned!
[23:40:36] [ForkJoinPool.commonPool-worker-4/INFO]: Session is valid

Addendum:
Clients utilize MultiMC's drag and drop feature to launch All of Fabric 3.

OS: Debian Buster

remove RER

Will try to test the server today / tomorrow with the following changes. Will try to respond back later tomorrow.

@Monster-Zer0 Removed RER on Server and user was still not able to connect. Is there some port that a mod uses that I should unblock in firewall?

User also had RER removed client side and was still unable to connect.

Make sure it's RER and not REI

There is no bug with the pack.. we run servers all the time.. and on Linux..

You need to do basic troubleshooting with your setup.

Modded minecraft doesn't do anything beyond normal minecraft when it comes to open ports.

Here, I'll throw a bone.. try removing xl packets.. then next try random patches.. then next make sure server tick is -1 then next try regenning the world..

Something is up with your setup and honestly only you can figure that out.

We ended up creating our own modpack. However, I suspect it has something to do with the ports. Thanks for the help.

Update: figured out the issue, rcon was set to the same port as native port. Setting it elsewhere fixed the issue.

sweet