We believe we have solved this problem. We thank Jan-Arne Sobania of HPI for his consultation.
This problem does not occur for standalone systems. It only occurs when you network MCPC/SCC systems together. And it's a simple fix, once you know how.
In systemSettings.ini, the MAC address of the first core is assigned as in sccFirstMac=00:45:4D:41:44:31. There are 48 cores. Each core gets sccFirstMac + its core number for its MAC address. When you assign sccFirstMac on another MCPC/SCC system, you must not use a number in that range. Making sccFirstMac unique for each system is not enough. The problem occurs when you forget that.