Unfortunately NC 2.5.6 (quite old) does not support Unity 5.5 or 5.6 (I am not sure how it even worked in 5.5 in the first place).
Support for Unity 5.5+ has officially been added in version 2.6.2 (released 5 months ago).
I suggest that you update your NodeCanvas to latest version from the asset store while still in a working version of Unity you had, and only after that update Unity to 5.6, so that everything is updated in the correct order.