ABB ASC880 on Modbus Communication Fault

Good day,

We have a number of industrial fans to be controlled by ABB AC500 PLC, (PM573), on Modbus, with VFDs ASC880s.
We get a strange Communication Fault from two of the VFDs. These two are in separate buses, and both are "in the middle" of other functional VFDs in their respective bus. Each bus has about 2 to 5 units on it.
As far as I can see, all parameters are the same on the "faulty" VFDs as on the ones which does not get faults, unless I miss something. The cables are tested OK, and the terminations are the same on the ones with faults as the ones without, (the modbus is connected to the drive-to-drive links on XD2D).
Does anyone have any idea on what can cause this communication fault to a drive in the middle of the line of working VFDs?
Some "hidden" parameter I don't find?
Something I totally missed?

Best Regards