WoCompleteCommand HTTP 404

Hello, I'm trying to automate the ability to complete a work order via API. See following code:

"{"
+ " "commandRequest": { "
+ " "type": "WoCompleteCommand","
+ " "WorkOrderId": "376508" "
+ " }, "
+ " "RequestId": "1234" "
+ " } "

This returns the following error:

 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml">
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"/>
 <title>404 - File or directory not found.</title>
 <style type="text/css">
 <!--
 body{margin:0;font-size:.7em;font-family:Verdana, Arial, Helvetica, sans-serif;background:#EEEEEE;}
 fieldset{padding:0 15px 10px 15px;} 
 h1{font-size:2.4em;margin:0;color:#FFF;}
 h2{font-size:1.7em;margin:0;color:#CC0000;} 
 h3{font-size:1.2em;margin:10px 0 0 0;color:#000000;} 
 #header{width:96%;margin:0 0 0 0;padding:6px 2% 6px 2%;font-family:"trebuchet MS", Verdana, sans- 
 serif;color:#FFF;
 background-color:#555555;}
 #content{margin:0 0 0 2%;position:relative;}
 .content-container{background:#FFF;width:96%;margin-top:8px;padding:10px;position:relative;}
 -->
 </style>
 </head>
 <body>
 <div id="header"><h1>Server Error</h1></div>
 <div id="content">
 <div class="content-container"><fieldset>
 <h2>404 - File or directory not found.</h2>
 <h3>The resource you are looking for might have been removed, had its name changed, or is 
 temporarily unavailable.</h3>
 </fieldset></div>
 </div>
 </body>
 </html>

I have ensured the WorkOrderId is correct and the user is correct. The string returned is the correct format as far as I can tell, see the returned string below:

{ "commandRequest": { "type": "WoCompleteCommand", "WorkOrderId": "376508" }, "RequestId": "1234" }

Permissions for the user account are all set to allow WoCompleteCommand. Not sure what the issue is, any help would be appreciated.