What device we have here? Some plcs might reject connections / requests (see openhab issue [here] (Better synchronization for openHAB and industerial controllers via Modbus · Issue #3537 · openhab/openhab1-addons · GitHub)).
Also, if the device accepts at most one connection at a time, commands will still be problematic with the current implementation. I invite you to test the new experimental version available at this thread
Edit : BTW, can you please paste longer logs on pastebin. It would be also nice to know whether you get errors if you remove the write commands, ie just polling enabled.
Best,
Ssalonen
edit 17.2.2016: fixed thread link