Hello rydeck, we are facing one in the same problem you related here.
We also have a KRC4 unit driving a KR 6 R900 sixx that, time to time, decide not to boot properly . . . . the problem is that w are a very small company, and we have fill or no attention at all of Kuka company representative in our situation.
Do you had success to solve this problem ? If so, could you share with us what to do ?
Best regards.