Not a lot anyone can do to help just from a screenshot, unfortunately.
Are you are using non-standard plugins? If so, move them out of the C2 folder, start a new project, save and close, add one of the plugins to C2. Load project and add plugins. Continue until error occurs, which will show you the problematic plugin.
If you are not using any non-standard plugins, then quickest way to resolve would be to post a link to a .capx and let us test it for you.
Also, what version of C2 are you using? Have you tried latest beta?
What browsers have you tested on?