Decon

This error occurs when IPS is trying to load a plugin or widget from an application and can't find it or it isn't working properly. I would start by finding out if it's a widget or a plugin, and working back from there. I don't know about in this specific case, but usually viewing the error log file can help get some insight on what is happening. Your error log can be found in /uploads/monthly_YYYY_MM/ folder or through the ACP. If that doesn't exist or tell you anything, then I would say to check that all your widget and plugins are installed properly.

It doesn't look familiar to any theme I've seen used by IPS, and browsing through the source code and custom stylesheet make's it look like it was custom made. They have also branded it as their own in the footer with the theme name, so I would say it's not available unless exported or ripped from their site. I may be wrong though.

The way IPS loads it's classes is by prefixing their original classes with a unique character (an underscore), then loading all the plugins/hooks and patching them over the original class, then patching an empty class with the proper name at the end. For example, IPS\_Output  would be extended by IPS\hook123 which would then be extended to IPS\Output (which would be blank), so that all the hooks (in the order they are installed I believe), are loaded before creating the right name for the output. By the looks of this error message, one of your hooks/plugins got deleted or uninstalled improperly, and now it can't find it while it's trying to patch it.
I've never experienced this issue before, but your best bet would be to check that all the plugins you have installed are installed correctly, and if you deleted any plugins, make sure they are actually out of your script so that IPS has no way of knowing that it ever existed in the first place. Look inside your /plugins/hooks.php file and look for the numeric ID 489 and see what it's looking for and what it's trying to override. Either delete that section of code in the file carefully to not mess up the rest of the file and then try again.