I’ve been contacted recently by a number of Zen Cart users who are experiencing on occasion, an error similar to the following while working in the administrative section of their sites.
Catchable fatal error: Object of class queryFactoryResult could not be converted to string in … html_output.php
The problem you are experiencing stems from leftover legacy code that is incompatible with PHP5.
Here are two links that will help you understand and correct the issue.
An interim workaround has been provided by the Zen Cart team. The link to the files which solve this issue are provided here.