That’s weird. I just did a test on a wordpress.com hosted blog using Twitter and Youtube embeds. Both worked.
I can review your dashboard, fix the embeds for you and explain what caused the error. This falls outside of the free support service I offer on WorP’Dev.
Let me know if you want me to look into this for you. It cost you only $30 (PayPal – satisfied or refunded) and all I need to start working on this is that you invite me to your blog as an administrator.