Which version WordPress are you using? Which version of PHP is on your server? Are you reporting this error with the latest version of the plugin (v .18.3)?
If the previous version was working for you, you can revert to it. Download previous version from http://wordpress.org/extend/plugins/list-category-posts/download/.
Otherwise, from the documentation (see FAQ) on the plugin’s site, it appears that this error is due to incompatibility with PHP 4. The plugin author, Fernando Briano, has addressed the issue over at wordpress.stackexchange.com (http://wordpress.stackexchange.com/questions/9338/list-category-posts-plugin-upgrade-fails-fatal-error/9340#9340).
Hope this is helpful.
Thanks daitya, you’re on the way to becoming one of List Category Posts’ most valuable user 😛
dandelionweb, as daitya said, I’ve addressed this issue and I included this error on the FAQ.
You should consider updating your server to PHP5 since WordPress will stop giving support to PHP4 in the upcoming version 3.1.
Thanks dandelionweb for reporting and daitya for the support 😀