After a clear installation (unistalled rhino V5 and rhino v6 - deleted Mcneel directory and Grasshopper in ApppData) I still have this problem when I simply start a python component (with import rhinoscriptsyntax string) and run start. Could anyone help me, please?
It looks like it might be similar to issues discussed here: (short version is that you might have a conflicting version of iron python installed.)
Thank you so much. Problem solved. i have unistalled my IronPython v 2.7.3 and now works like a charm.