Error message on top of every page after installing new version of PHP

I have three websites built using either the Unicon or Unicon Child theme.
http://www.stephanie-watson.com
http://www.plumlines.net
http://www.cedarcomposters.com

I updated to the newest PHP a couple of months ago, and now on two of the sites (stephanie-watson.com and plumlines.net) there’s an error message at the top of each page. Here’s the one from the plumlines site:

Warning: Parameter 2 to WPE\Site_Preview::the_posts() expected to be a reference, value given in /homepages/35/d347541676/htdocs/plumlines-v3/wp-includes/class-wp-hook.php on line 286

And the one from stephanie-watson.com:

Warning: Parameter 2 to WPE\Site_Preview::the_posts() expected to be a reference, value given in /homepages/35/d347541676/htdocs/swatson-v2/wp-includes/class-wp-hook.php on line 286

The sites are built on WordPress. I believe I am using the latest version of Unicon (v 2.6.1) and Unicon Child (v 1.1). I’m not very I’m not sure where to begin in solving this problem. Your help would be appreciated.

Thanks!
Stephanie

Ticket