Does this mean something in the database is mapped to a D: drive?

I was recently approached by a client whose wordpress site had been hacked. The hack caused traffic from search engines to be hijacked to cuteonbags.com.

I narrowed down the malware in question and found what sounded like a relatively easy fix, but the problem is exacerbated by a number of issues.

1. The client is using GoDaddy on Windows, crippling my ability to access, search, rename or otherwise manipulate the files behind the scenes. I am rebuffed at every turn. I can't rename plugins folder to disable them, can't modify permissions, neither in FTP apps nor in the GoDaddy control panel, and GD's support docs simply lie. Nothing is as they say.


2. The client went years without ever updating wordpress or any plugins. Maybe this is why.

After affirming the client had backed up the files and database (only after the hack occurred, of course), I decided to bite the bullet and upgrade to wordpress 4.1.1 from the admin. After doing so, and trying to access the admin area, I get:

Fatal error: Call to undefined function wp_json_encode() in D:\Hosting\10541703\html\wp-admin\includes\misc.php on line 668

Call me crazy, but is that path pointing to a hard drive? That can't be right... or is it?

Is this error originating in the database? I realize I should probably start the process of a manual reinstall from a fresh copy of WP (if the client okays it), but the nightmares of doing so on a GoDaddy Windows server are looming, especially considering even the simplest routine operations are hobbled by their (plesk?) control panel, outright hostility to FTP access (forget the shell), horrible support, and inaccurate documentation. If the process so far is any indication, I may be tilting at windmills here.

Angst aside, an anyone shed some light on this error, or what might have gone wrong in general?