For Juniper Networks EX2300 and EX3400 Series Switches, in some versions, the user will not be able to connect to the switch via console, getting logout as soon as credentials are entered.

On console when you entered username and password, you will login and log out immediately getting back to the login prompt , following will be seen on console,

login: Admin

Password:

Last login: Mon Jan 1 10:10:01 on ttyu0

--- JUNOS 20.4R3-S2.6 Kernel 32-bit JNPR-11.0-20211117.c779bdc_buil

FreeBSD/arm (nyc-3400) (ttyu0)

login:

As workaround you can connect via SSH and delete/deactivate "set system ports console log-out-on-disconnect".

If the switch cannot be accessed via SSH, you may access recovery mode via console connection in order to change the configuration and to commit. To perform this process, you need to power cycle the switch and to stop the boot sequence with the steps below:

1. Reboot the switch and interrupt the boot process by pressing "Control + c" when you see the following Main Menu:
 

Main Menu

1. Boot [J]unos volume
2. Boot Junos volume in [S]afe mode

3. [R]eboot

4. [B]oot menu
5. [M]ore options  <---press 5

Choice:
Options Menu

1. Recover [J]unos volume
2. Recovery mode - [C]LI  <---press 2 
3. Check [F]ile system

4. Enable [V]erbose boot

5. [B]oot prompt

6. [M]ain menu

Choice:
Booting Junos in CLI recovery mode ...
|
/packages/sets/active/boot/os-kernel/kernel data=0xd0c000 syms=[0x4+0x4cde0+0x4+0x6d35f]
/packages/sets/active/boot/os-kernel/ex3400.dtb size=0x1ce1
/packages/sets/active/boot/junos-modules/fips_core.ko text=0x1074 data=0x247 syms=[0x4+0x600+0x4+0x40f]
...
...
...

<Some boot messages omitted>

Starting CLI ...

warning: This chassis is operating in a non-master role as part of a virtual-chassis (VC) system.
warning: Use of interactive commands should be limited to debugging and VC Port operations.
warning: Full CLI access is provided by the Virtual Chassis Master (VC-M) chassis.
warning: The VC-M can be identified through the show virtual-chassis status command executed at this console
warning: Please logout and log into the VC-M to use CLI.

2. Delete/deactivate "set system ports console log-out-on-disconnect". Example of deleting this stanza below:

{linecard:0}
user@switch> configure
Entering configuration mode

{linecard:0}[edit]
user@switch# delete system ports console log-out-on-disconnect

{linecard:0}[edit]
user@switch# commit
commit complete

3. Exit configuration mode and reboot the switch.

user@switch# exit
Exiting configuration mode

user@switch> request system reboot
Reboot the system at now? [yes,no] (no) yes

After reboot, you should be able to login to the switch normally. For permanent fix you can go to fixed versions under PR1680408