PHP Error when selecting a product category from the store front page

I am getting the following error when trying to select a product category from the default list present on the store front page:

Catchable fatal error: Object of class WP_Error could not be converted to string in /hermes/waloraweb078/b350/moo.blueridgecrorg/CSEMS/wp-includes/formatting.php on line 2583

The store front page is: http://www.csems.org/store/