[Plugin: Constant Contact for WordPress] Fatal Protocol Error

I see other posts about the same topic, which is the fatal protocol error in “wp-content/plugins/constant-contact-api/class.cc.php on line 1913”

However, I’m unclear on whether this is a problem on my end or the plugin (or Constant Contact). Some people seem to have it resolve itself on its own, I’ve seen the plugin author mention they needed to upgrade the plugin, and I’ve seen others suggest it’s a server issue.

Where do I start the troubleshooting?

Thanks. 🙂