You may see this message on some servers when you try to save an event. It arises because you have configured JEvents to check for overlapping events when you save a new or changed event. If you don't need this on your site then disable this option - if you do want to block overlapping events then please read on:
It is usually caused by the fact that your server cannot run the script components/com_jevents/libraries/checkconflict.php usually because you have installed a security setting that block all php scripts apart from the main index.php script.
This could be caused by:
The checkconflict script is a slimmed down version of the core index.php script that doesn't load a lot of the core Joomla CMS features like the menus and the main router. This makes it a LOT faster to check for overlapping events before passing control of the script on to the main event saving routines
If you see this issue in RSVP Pro then please see this article.