EASY SOLUTION (from another post)
No default menu is set in the Main Menu. Go into your Admin page/control panel and go to the Menu Item Manager for the Main Menu. Click the check box next to the Home page at the top of the list. Then click on the yellow star icon in the upper-right corner.
ALTERNATE SOLUTION
So I set Debug Info to Maximum and examined the function call stack. After further analysis I noticed that the error was getting triggered when trying to call the dispatch() function for a component (which one I could not tell). It calls the renderComponent() function which does a check on the component name, if it is empty then it raises a 404 Component Not Found Error. For whatever reason Joomla seems to be calling dispatch() on a component with an empty name (possibly because a component is disabled or the FrontPage is broken/missing,) which it should NOT do, and generates the error. I put a very simple if statement in the code which fixed the problem!
In the "includes" directory in the root Joomla directory there is a file called "application.php" Locate the dispatch() function (at line 89). Right before the call (at line 124) to JComponentHelper::renderComponent() put in this "if" statement like below. Make sure it looks exactly like this.
if(! empty($component)) {
$contents = JComponentHelper::renderComponent($component);
$document->setBuffer( $contents, 'component');
}
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