KCB is Control Bus... so drives etc are on it. if any single node has loose connection, bus will fault out. one thing i saw few times is loose connection at the data cable between KRC and RDC (X21-X31) due to missing O-ring, but the same could be on any of the KPP/KSP as well.
i would suggest to make fresh KRCDiag at the moment this happens and send it to KUKA Support. there may be things to check like firmware and software compatibility etc. Perhaps something need to be updated.