Here’s more information:
As of February 20th, 2014, there is an internet-wide Google/YouTube bug for forcing HD playback. It affects all WordPress and non-WordPress sites.
We’ve notified Google of the problem and hope to hear that they’ve fixed it soon. In the meantime, turning off the HD option in the plugin’s settings page should solve your problem.
Also, if you have the time and a gmail address, you could click on the star for this exact Google support issue at Google Code:
https://code.google.com/p/gdata-issues/issues/detail?id=6009
The more people star it, the more resources Google puts into fixing their error.
Finally, all users of this plugin will be alerted when we hear back about a fix.
Thanks,
EmbedPlus Team