The error above is a 500 error meaning that the server encountered an unexpected condition. This could be a code error in a Template file.
Errors on the frontend are presented in three ways:
500 (internal server error)
Blank / not loading
404 (Page not found)
A 500 error will show a generic error message like the one shown above or the full error stack trace if debug mode is enabled. The full error message can be found in the Umbraco Logs.
A blank page or a page that is not loading is likely due to bad rewrite rules ending in a rewrite loop. Make sure you do not rewrite anything on these paths:
You can see examples of how to set up rewrites correctly in the Rewrites on Cloud article.
You should also check your console log in your browser to see if something is failing to load. Finally, you can check the umbracoTraceLogs and see if any errors are thrown!
A 404 page could also be rewrite-rules. See above for what to check for. Other than that make sure your site and content structure is set up correctly. If you try to access a content node with no template related to it will throw this error.