The latest C2 stable release r178 is downloadable from the link in the footer. It would definitely be a good idea to upgrade when possible. You don't say what version of C2 you are currently using.
What browser are you testing on?
Can you create a new simple .capx that shows the problem and attach it to your post, so we can try it for ourselves?
Google throws up some interesting stuff on this error, so it may not be a C2 problem at all, but without a .capx that we can all test for you, we are just guessing.