I see this problem occasionally, too. One thing that sometimes solves the problem sometimes is opening the project properties, making a minor change, and then saving the changes. It seems that this somehow triggers something to be refreshed.
There’s a discussion about this problem here:
http://forum.inductiveautomation.com/t/codecregistryerror-when-importing-json-library/
I’ve also talked with Inductive Automation support about the issue, but we haven’t had any luck getting to the bottom of it.