It sounds like the upload of 1.6 fails in the first place!
The NeDi init only updates the DB scheme to the current version. If the upload fails, your still running 1.5, which can only perform an update from 1.4…
To update NeDi via browser the following prerequisites have to be met:
On your server in php.ini or php-fpm.ini, make sure the NeDi archive can be uploaded:
post_max_size = 128M
upload_max_filesize = 128M
Also if you’re running nginx:
All files need to be writeable by webserver:
chown -R www-data.www-data /var/nedi
Now you can try again and you should get a green status line after uploading 1.6.