[Plugin: Simple Tags] 500 server error when simple tags is present

Hi, I am running WordPress 2.8 and Thesis 1.5.1.

I have many plugins running fine in this setup. However when simpletags in installed I get a 500 server error, every time. When I remove the plugin the error goes away.

Anyone have a suggestion how to pinpoint the issue?

Thanks.

http://wordpress.org/extend/plugins/simple-tags/