Do not use the WordPress automatic upgrade process on this plugin | Peter's Custom Anti-Spam for WordPress | Forum

While the WordPress automatic upgrade process can be handy, it does not work well with this plugin. This is mainly due to the fact that the WordPress automatic upgrade process wipes away the plugin's directory before extracting the new plugin. In particular, if you ever add custom fonts to be used by this plugin, they must be stored on the file system, and thus they are removed when the WordPress automatic upgrade process runs.

Therefore, the recommended upgrade process is to download and extract the plugin manually from its page on this site or on the WordPress plugin page.