You should be a little more specific about your whereabouts.
The error message "Assertion failure...", do you get it when you preview an opened project ? a new blank project ? when you launch Construct2 editor ?
The message leaves me to believe there's a missing plugin, maybe a difference between the version of the project and the version of the editor.
Either try to delete (rename to keep events) the current mouse object of your project and create a new one. (the one of the current editor, so up to date).
Try to preview again to see if any change occurs.
If that fails, maybe you installed the current C2 editor over an old installation/dezip. I'd suggest then to uninstall C2 (delete the folder manually if no uninstall is present), redownload it and reinstall it.
Also think about updating your browser as well as telling us which "brand" it is ? (IE, firefox, chrome, safari... ?)