I found that this is caused by incomplete neighbor settings between MCU and VCS-C.
The other day we had some test to check relationship between embedded gatekeeper function of MCU and VCS-C neighbor.
After that test, we disabled the embedded GK function on the MCU but we did not disable VCS-C neighbor.
So, VCS-C kept trying to make neighbor connection between the non neighbor GK(=MCU) but the MCU could not establish connection because it was no longer GK.
Finally, by deleting neighbor zone for the MCU, error messages stopped.
Kotaro