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I don't know why it is happening, but when I go to list setting where we choose if it is a double or single opt-in when I hit save button it simply go to a blank page de final "sendy/includes/list/edit.php" and not is saved.
Try re-uploading Sendy:
Download a copy of Sendy from http://sendy.co/get-updated with your license key (use the form at the top left of the page to download Sendy).
Then re-upload all of Sendy's files to your server (except /includes/config.php and /uploads/ folder) replacing the old ones.
Make sure all necessary files are uploaded.