@FredericMa, please try the latest version from today. It should include the fix I described some time ago. Sorry for the delay, been quite busy with non-openHAB stuff 
@Nanna_Agesen, I notice that in the old binding you do not have writemultipleregisters enabled, but with the new binding you do have writeMultipleEvenWithSingleRegister=true. Perhaps try removing the writeMultipleEvenWithSingleRegister or set it false (the default)? Perhaps your slave does not accept the function code “write multiple” (as indicated by the error code!).
@all :
I have put some effort to documentation, you can read it here: https://github.com/ssalonen/openhab2-addons/blob/modbus-openhab2-native-binding/addons/binding/org.openhab.binding.modbus/README.md
As you can see, the new binding docs are in the beginning, and old binding docs are now just appended in the end. Naturally this is just WIP, and eventually you would have nice single doc.
Also note that now there is new thing type data to replace read, write and readwrite things. The old things are still there so you can compare the two, hopefully still working as before. Please note that the read/write addresses with data are “absolute”, not relative to poll start as with read and write. Check docs for more explanation on the parameters.
The original concept behind data is described here: Modbus openHAB2 binding available for alpha testing . I have also implemented support for the JSON transformation output as described in the post to cover the more complex use cases.
Please, try it out and check if it feels more natural/simple now? I think I like the new structure better at least