The "Last Error" field includes 3 bytes (Hex) that are NetID, Function and Exception Code.

NetID
For easy understanding which request gets problem,
the NetID may be keeped as original one instead of return NetID of the Gateway.

Function Code
There is error when the returned function code is larger than 0x80.
For example: Request function=0x01, Retured function=0x81.

Exception Code
Most exception codes are defined in Modbus protocol as followings,
but we cannot guarantee that all slave devices are fully following the standard.
You may need to check the user manual of the slave devices about its exception.

Exception CodeDescription
0x01Illegal Function
0x02Illegal Data Address
0x03Illegal Data Value
0x05Acknowledge
0x06Slave Device Busy (Gateway reports)
Too many requests are sent/queued in the same time.

Suggestions:
Configure the client software with a larger (slower) scan-time/timeout setting.
0x07Negative Acknowledge
0x08Memory Parity Error
0x0AGateway Path Unavailable
0x0BSlave Device Timeout (Gateway reports)
No response from slave device.

Suggestions:
Try larger Slave Timeout setting.
Check serial port signal (resistor), wiring, NetID and baud rate/data format configurations.
Exception CodeDescription (Non-standard Exception)
0x41Invalid Modbus message from slave device (Gateway reports)
Wrong data is received (not a standard Modbus message).

Suggestions:
Check serial port signal (resistor), wiring and duplicated NetID.
0x42Wrong data length in received message. (Gateway reports)

Suggestions:
Try larger slave-device-timeout setting.
0x43CRC Error on slave device response (Gateway reports)
Wrong data is received (not a standard Modbus message).

Suggestions:
Check serial port signal (resistor), wiring and duplicated NetID.
0x4BData Timeout (Gateway reports)
Response is under receiving but timed out.

Suggestions:
Slave Timeout setting is too short. Have a larger value instead.

Related Software:
WireShark (freeware):
When troubleshooting, use this to get the Ethernet network communication log for reference.

Modbus Master Tool: (freeware)
The is an Modbus TCP/RTU/ASCII master test program. Useful when testing slave devices.

Modbus Utility: (freeware)
This software includes Modbus TCP/RTU client (master) program too.