BigIP LTM Standby device is showing offline on the active device

I worked with F5 support and they took three days to troubleshoot the issue. Finally we ended up re creating the HA and that is when we found out the interfaces used for the HA is having a problem. When we used the management IP for failover, everything worked good. But when we use the HA interface IP, the standby device was showing offline. Not sure if the cable that is used to connect the HA ports is the problem. Support was unable o determine that. Thanks Piotr for all the different solutions you gave.