I do not say you are wrong, it may be possible that the network connection through the service port is the issue. However it is difficult to believe, since all functions are correct through this port. I can connect both via FTP and EGM (UDP) and I receive all trajectory data.
I have static addresses and I have only one pc connected to the robot, but it is true that both are connected to a switch in between.
In any case I cannot see how this can affect the internal clock of the robot controller.
ABB has suggested me to change the CMOS battery in the controller. We will try this and also connecting only the PC to the service port and I will let you know the result.