TaskCreationOptions.LongRunning causes Exception. Does anybody know why?

Updated to b4.6 this morning and my program stopped working because of the following line of code:

task = new Task( () => { Run(); }, TaskCreationOptions.LongRunning );

An exception was thrown by a TaskScheduler.
Couldn’t create thread. Error 0x2

Got rid of>>> TaskCreationOptions.LongRunning <<<and everything is working again

Does anybody know why? It was not causing a problem under b4.5 and below.
Thanks, Paul