Not all or more than the expected number. . .

I have a 3-phase system with 6 Quattros, which has been working for about a 6 months. Now, whenever phase L1 is loaded it immediately shuts down with a series of VE Bus errors: not all or more than the expected number of devices, Slave has assumed master status, Cannot transmit data etc.

Both Quattros on L1 have been changed, but the fault remains.

Any ideas?