We have ex4600/QFX as a virtual chassis that the VC-Ports are not working after upgrading to the 18 JunOS version.

Wanted to connect redundant vc-port on et-0/0/26 on both devices but it didn't work.

The port will come up and goes down frequently.

Then configured other ports as vc-port and had the same behavior with that, but after 10mins or so the port came up.

Ports work as Network ports but not vc-port.

This issue is tracked by PR1555741

After upgraded to 18.4R2-S4 or later release on EX4600 or QFX5100 VC, the QSFP+-40G-LR4/LX4/BXSR optics used as the VCP might be down and not be up again, then the fxpc process might get high CPU usage (e.g. 60-80%) during this issue, the VC might be rebooted 2-3 times and the fxpc process might crash too.
The following log messages might be seen when running into this issue.

> show virtual-chassis vc-port 
fpc0:
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
0/24        Configured         -1    Up           40000        1   vcp-255/0/24
0/25        Configured         -1    Down         40000                        	<<<<<<<<<<<< Issue state
0/26        Configured         -1    Up           40000        2   vcp-255/0/26
0/27        Configured         -1    Up           40000        3   vcp-255/0/27

 
fpc1:
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
0/24        Configured         -1    Up           40000        0   vcp-255/0/24
0/25        Configured         -1    Down         40000                        	<<<<<<<<<<<<< Issue state
0/26        Configured         -1    Up           40000        3   vcp-255/0/26
0/27        Configured         -1    Down         40000				            <<<<<<<<<<<<< Issue state

fpc2:
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
0/26        Configured         -1    Up           40000        0   vcp-255/0/26
0/27        Configured         -1    Down         40000				            <<<<<<<<<<<<< Issue state

05-11-2023:KB created.

PR1555741