CAN Bus-C Failure?

I physically disconnect pins C1:3 & C1:10 (bus B) on an MD4-10.  After startup, the message on the screen was for bus -C failure.  All channels connected to bus C were in a failed state.  I reconnected and communication was re-established.  I then disconnected pins C1:4 and C1:9 (bus C).  On startup, there was no error message, but no data coming in on C.  I disconnected these pins directly at the module, so no chance of a wiring issue.

Originally, the devices on the two buses were configure on bus C only.  They were later re-configured to be on the separate buses.  All the CAN frames for these two buses are in external functions.  This may be related to the timer issue I found related to the external function.

https://forum.iqan.se/communities/1/topics/1381-timer-designsimulate-5-bug

If I am missing something, please someone enlighten me.