The easiest way to check for invalid characters is to go to the W3C Markup Validator and type in the URL of your WordPress site.
If you get a response such as “… one or more bytes that I cannot interpret as UTF-8″ that is most likely what is causing the app to have trouble with your site.
Often, invalid data errors are caused by theme and plugin conflicts. You can check for these by switching to your default theme and deactivating your plugins. You can identify the offending the theme or plugin by activating one at a time and then retesting the app.