This problem seemed to appear randomly after I upgraded to 4.1.2 (from 3.3.3).

But, I've now concluded that its a problem only during asynchronous notifications where cron_exec.php is invoked from a cron job.... works when I execute pending jobs from the backoffice.

For now I've switched the immediate notification....so haven't dug further, but my initial thought was that its a problem somewhere with the twitter plugin and the url shortener?

The Dreamer.