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FXM Add External Site Throws "System.InvalidOperationException" Error

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FXM Add External Site Throws Error

version: Sitecore 8.2 update 1.

I'm getting an error whilst adding an external site to FXM. when i complete the settings and click Save, the page appears to save but doesn't refresh the page as it usually does. looking at the chrome console, i get the following error:

http://localhost/sitecore/api/ssc/domainmatcher/service?id=%id% 500 (Internal Server Error) - entityservice.js

Uncaught Error: {"Message":"An error has occurred.","ExceptionMessage":"There is no method 'Filter' on type 'Sitecore.ContentSearch.Linq.QueryableExtensions' that matches the specified arguments","ExceptionType":"System.InvalidOperationException","StackTrace":" at System.Linq.EnumerableRewriter.FindMethod(Type type, String name, ReadOnlyCollection`1 args, Type[] typeArgs, BindingFlags flags)\r\n

using a clean build works but this particular version just seems to throw that error everytime. any ideas that could lead me to resolving local setup issues causing this error?