How to fix "'System.AggregateException' occurred in mscorlib.dll"

As the message says, you have a task which threw an unhandled exception.

Turn on Break on All Exceptions (Debug, Exceptions) and rerun the program.
This will show you the original exception when it was thrown in the first place.

(comment appended): In VS2015 (or above). Select Debug > Options > Debugging > General and unselect the "Enable Just My Code" option.

You could handle the exception directly so it would not crash your program (catching the AggregateException). You could also look at the Inner Exception, this will give you a more detailed explanation of what went wrong:

try {
    // your code 
} catch (AggregateException e) {

}

The accepted answer will work if you can easily reproduce the issue. However, as a matter of best practice, you should be catching any exceptions (and logging) that are executed within a task. Otherwise, your application will crash if anything unexpected occurs within the task.

Task.Factory.StartNew(x=>
   throw new Exception("I didn't account for this");
)

However, if we do this, at least the application does not crash.

Task.Factory.StartNew(x=>
   try {
      throw new Exception("I didn't account for this");
   }
   catch(Exception ex) {
      //Log ex
   }
)