I had a major problem with 4 sites this morning, my 2 sites and 2 of my clients sites. I was getting 500 server errors on the front end but we could navigate the WP admin fine. All sites were on the same host but several other of my clients sites on that same server were not affected.
After panic driven troubleshooting we realized that Social Media Widget was causing the issue but we didn't know why. I had not updated anything on these sites since last week. But the host had updated PHP on the server to 5.4 sometime last week. I had all sites aggressively cached so that's probably why the problem didn't surface until this morning.
The thing is, an identical copy of the site on my localhost running PHP 5.3.x runs OK but as soon as I activate SMW 3.3 on the live site it goes down (public side). Same other plugins and theme activated.
Maybe there's code in your plugin that no longer works with PHP 5.4?