The requested page "/mywebsite/?q=admin%2Fcontent%2Fnode&render=overlay" could not be found
The url you have posted translates to

admin/content/node?render=overlay

This is how the overlay works, which displays a regular page, but adds a param to tell drupal it should be rendered as the overlay (no header, footer etc is rendered).

The problem could be that overlay is disabled, or that the url isn't encoded as it should.

If you use clean urls, you shouldn't see the ?q=...