Read maps having an error status are listed on this page. If nothing is listed, no errors have been detected at this time. Internal error codes are translated into descriptions here for convenience.

Errors will be automatically reset by the next successful processing of the indicated map.

The most common exception code returned by a Modbus slave is "Illegal data address". That is the official Modbus protocol designation. What it means is that you attempted to read a register that does not exist. Check to see that you did not enter 40001 for holding register #1 - 40001 is short hand for holding register #1 and you must enter just "1" while selecting "holding register".