Screenshots.com use a browser with Webkit engine (but it's impossible to determine which version), but since DomainTools uses Python most of the time (they say it), there is a high probability that "Webscraping"
http://docs.webscraping.com/introduction.html
is used to generate screenshots... Gantry might be generating errors because of a badly configured user-agent.
The "CHANGELOG.php" of Gantry is protected, so it is practically impossible to make further investigations about the affected versions... However, websites using Gantry for Wordpress do not seem to be impacted by this kind of bug.
I hope the bug is a minor bug : The problem could be "superficially fixed" by preventing errors display (if it's just a problem at the server level), BUT exposer all the directory structure of a server is never à good thing.