I've never seen this error, so I was assuming toJSONString was something you had added. As far as I know that is never used in our extension, so I'm at a loss as to where that error is coming from. Do you have other extensions installed that might be causing a conflict?
I just did a search on the web for "Dreamweaver toJSONString error" and found some posts implying this might be another winfilecache .dat error. Try deleting your winfilecache with DW closed and it may correct this problem.