Error Message "Request Entity Too Large" when trying to save menu

When I try to save the main menu, I get the error message:

"Request Entity Too Large
The requested resource
/wp-admin/nav-menus.php
does not allow request data with POST requests, or the amount of data provided in the request exceeds the capacity limit.
Apache/2.2.29 (Amazon) Server at 54.77.37.yyy Port 80
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However, editing and saving the menu worked perfectly a couple of weeks ago.

Any idea on what might be the issue?

Thank you!