The physical interface goes to an up/down state as soon as it's added to the LAG.

ex4100 ae0 <=======> ae0 ex4550

ae0: interfaces ge-0/0/0 and ge0/0/1

The "show lacp statistics interface" command shows that no messages are exchanged between the remote devices.

user@host> show lacp statistics interfaces ae0  
Aggregated interface: ae0
  LACP Statistics:    LACP Rx   LACP Tx  Unknown Rx  Illegal Rx 
   ge-0/0/0        0    0      0      0
   ge-0/0/1        0    0      0      0

Via CLI run the commit full force hidden command:

host> commit full force

• This command above will force the switch to reload the entire switch's configuration again.

On a virtual chassis try to perform a switchover:

host > request virtual-chassis routing-engine master switch

• This command above initiates a switchover, making the specified device the master routing engine of the virtual chassis.

Reboot both Devices:

request system reboot

• Wait for the devices to reboot and become operational again.

Validate Statistics:

• Once both devices have been rebooted, use the show "show lacp statistics interface" on each device to verify that transmitted and received packets are being counted.