Error Loading the available lists failed

List Lookup -   Error Message (using Nintex WorkFlow and Nintex Forms 2013) 

- Loading the available lists failed.  Please check the URL you have provided is a valid URL within the current site collection.  

 Any idea what i can do to fix this or what could possibly be the problem.   Note When I change the connection status to Not Connected  the Source Share Point site is displayed with "0087F8D0-9C77-4FD1-850E-37125140AA6D"  and the source List display message is  -“ Loading Please wait...”

 Any assistance will be highly appreciated.

Thanks

Carol