Cannot save changes to settings

When I click ‘Save Changes’ in the lower right corner, the page reloads with

403
Forbidden
Access to this resource on the server is denied!

I tried changing the video player size through the php file at
/wp-content/plugins/youtube-embed-plus/youtube.php
but they did not affect the video size.
I also tried setting permissions to the file for write:everybody.

Because I cannot save changes, I cannot change the API key after the first install!