If I recall correctly the modbus 1.x binding log messages are sometimes missing important information (version 1.x binding: 1 & 2; vs; version 2.x binding: 1 & 2)
If you still have stamina to try this out further, I welcome you to try out the version 2 of the binding: Modbus openHAB2 binding available for alpha testing .
You can also try to use 3rd party tool to poll the slave, e.g. modpoll or pollmb