I think all of this is related to server configuration. None of the notices you are getting at the top are causing any problems. It looks like you have “magic_quotes_gpc” enabled in your php configuration (which is no longer supported past php 5.3). Not exactly sure how this happened unless you upgraded from php 5.something.
I would either install php 7 from scratch or updated your php.ini file or htaccess to prevent this behavior (https://stackoverflow.com/questions/17455377/why-isnt-disabling-magic-quotes-in-php-ini-taking-effect)
Once you prevent the Fatal error from happing things should be fine.
Also, keep in mind that the server needs read and write permissions to to private folder under public root.