The problem could be caused by a couple of things, but I cannot tell without looking into it.
Here is what you should try:
1. back up your website to be sure anything happens you have a copy of the site which surely works
2. deactivate the plugin(if you can't from the WP administration area then just rename the wp-content/plugins/quick-adsense folder to quick-adsense-test via FTP and this will deactivate the plugin)
3. from the administration area try to reactivate it and check if this solves the issue.
4. if the problem still persists then deactivate the plugin and delete all the plugin files via FTP(just this plugin's files) and reupload the plugin files from your computer(important: you have to upload the files for the same plugin version you had)
5. activate the plugin and check if the settings work
6. do the modifications described in the above post once again and make sure you don't modify anything else
Let me know if you managed to solve the problem.