I could reproduce your error. You have to delete the whole line and then save your crontab. Now it should do it without error. Then you have to add the line again:
15 * * * * /usr/local/bin/php -f /usr/local/www/nextcloud/cron.php
Do not copy it but write it.
Then save your crontab again, now hopefully without error.
The main problem is, you have first to save your crontab without error, even if it is empty but for the leading comments. Once saved without error you can modify it again.