Looks like you have messed something else, because the path in the error message looks wrong to me:
file_put_contents(/meta/services.json)
IMO, it should be something like:
file_put_contents(/var/www/webapp/app/storage/meta/services.json)
Because, if you have done the symlink correctly you did something like
mv /var/www/webapp/app/storage /mnt/ssd/
ln -s /mnt/ssd/storage /var/www/webapp/app/storage
PHP is trying to write a file in the root /meta folder, where probably it doesn't have right.