"Page not found" or "Forbidden" when submitting form Is it a problem with Apache? I have some reseller hosting and I am running Drupal 7 sites on several different accounts there.

I recently noticed a problem when updating one of them. It happens when I am editing content or changing admin settings. 


I go to an admin page - e.g. my.site/admin/people/permissions. 
I make a change and hit "Save". 
The page submits and reloads. The address
bar still says my.site/admin/people/permissions, but the page content
is:


  Page not found
  The requested page "/admin/people/permissions" could not be found.



I tried changing .htaccess files, disabling modules, etc, but nothing worked. In the end I did the following:


Created a new hosting account on my reseller hosting
Installed a fresh copy of drupal-7.58
Created a new database


When I repeated the actions above, I got the same results, except this time the error message was the following.


  Forbidden
  You don't have permission to access /drupal-7.58/admin/people/permissions on this server.
  
  Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.


I then checked another account on the same reseller hosting. It's running Drupal 7.54 and showing Page not found. 

I didn't have the problem a few weeks ago, so I am wondering if my hosting company has changed something in Apache which is causing this problem (really clutching at straws here).

I am using:


Apache 2.2.32 
PHP 5.6.30 
MySQL 10.0.34-MariaDB


Can anyone help?
Based on the given error message, the issue is definitely about file and/or directory permissions or ownership. Make sure all the files and directories belong to the username under which your website runs. Also fix the permissions per https://www.drupal.org/node/244924.