Hello,
Have you tried to remove all the content of your "core/cache" folder ?
Deleting the core/cache folder content is safe, I do it every day in production website without any problem.
All the cache will be automatically rebuild.
What does the error page say ?
Its the MODx Error page that I have created in the manager previously - e.g. the custom error page
Is it the 404 page or the Unauthorised page? If it is the 404 page is it possible that the /manager folder, or it's contents, could have been moved or renamed, resulting in a genuine 404 error?
Check the core/cache/logs/error.log file.
Also, look at the content of the /manager/index.php file to see if it has changed.