Cannot update to 14 with this error
Could you perhaps show the permissions for /usr/share/nginx/hmtl?
Can you create a whoami.php with the following:
<?php echo get_current_user(); ?>And then upload the whoami.php to the updater director and then access it from a web browser?
The /usr/share/nginx/html/nextcloud/ directory and files need to be writable as the same user returned by get_current_user() or writable by a group that user is a member of.