can anyone help with this please?
I have seen similar errors on my system, when the PLC replies with nonsense. The HW of mine has a limitation on how many registers can be queried on one call. Error what i have seen is "Exception from modbus master: Unsupported IP protocol id: 1"
The topic about it is here http://mango.serotoninsoftware.com/forum/posts/list/549.page
If this happens all the time and not just randomly, you could take a capture of the messages with wireshark. It would help on checking what are the message content.