System.AggregateException

Well, as its name suggests AggregateException, is just a container for one or more exceptions which may be thrown when using PLINQ or TPL. 

As such exceptions may be thrown on different threads and may also occur concurrently, the system automatically catches and rethrows them within an AggregateException wrapper to ensure that they all get reported in one place. The exceptions themselves are exposed via the InnerExceptions property. 

You can catch an AggregateException and check which exceptions it actually contains with code such as the following:

try

{

    // perform some parallel operation

}

catch (AggregateException aex)

{

    string messages = "";

    foreach(Exception ex in aex.InnerExceptions)

    {

        messages += ex.Message + "\r\n";    

    }

    MessageBox.Show(messages);

}

So I suggest you do this to see what is causing the problem.