YouTube Advanced by Embed Plus not working right after upgrade to WP v3.9.1

I maintain several websites, on two of which I use the YouTube Advanced video posting plug-in by Embed Plus. The plug-in worked flawlessly for me for months, then recently began some bizarre behavior on the site that I post to most often. Videos could not be centered, and would not vertically align properly, especially if the embed code was in the interior of the posted text rather than at the end. Even when the embed code was at the end of the post, the video image would be displayed on top of the info line at the bottom of the post.

At first I suspected a plug-in anomaly, and I deleted the plug-in and then installed and activated it again. The results, however, were no different. This caused me to suspect WordPress, and even though Embed Plus says the plug-in works with WP v3.9.1, I decided to test this for myself.

To do so, I logged in to another of my sites that I do not use much. This site was still using WP v3.8.3 and v4.4 of the Youtube Advanced video plug-in. I tried a new posting using the same video with the same parameters, and it worked perfectly. I then deleted the post, updated the plug-in to the latest version from Embed Plus (v4.9) and repeated the procedure. Again the video displayed and functioned perfectly.

Lastly, I deleted the second post, updated WordPress to the current v3.9.1, and repeated the procedure again. Now I got the exact same screwed-up result that I had been getting on the original website.

This seems to confirm that, in spite of what the Embed Plus techies think, the latest YouTube Advanced version is not fully compatible with WordPress v3.9.1.

One of these sites uses the unaltered WP Twenty-Ten Theme, and the other uses a Child of the WP Twenty-Twelve Theme. My browser is Firefox, but I get the same dysfunction when I view the sites with AOL/IE. Other than that, I cannot think of anything that can possibly be contributing to this problem.

So, has anyone else had this issue, and does anyone have thoughts on how it can be rectified?