bukge on "Error 500 on appearance and plugins pages"

Error 500 ISE is very generic and can be caused by various problems including server errors. Here are a few things you can try to solve it:
- Your first step should be checking your site's error logs for a more specific error message. If you need help locating them, ask your hosting provider to help you with that.
- it could be problem with the plugins, try resetting the plugins folder by FTP or phpMyAdmin.
- it could be a problem with the theme. Momentarily, try switching to the default WordPress theme via FTP or whatever file management application your host provides by renaming your current theme to rule out any theme specific issue.
- It may also indicate a problem with the configuration on the server. Usually your hosting company tech support can help.
- If the above troubleshooting steps fail to resolve the issue, try manually re-uploading all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones. Read the Manual Update directions first!