Browsers show site fine but bots/scripts get an error 500?

So I am getting ready to launch my site http://www.sensidynegasdetection.com and am very happy with our first use of MODx as a CMS. However, I noticed when I was trying to get Google to find my sitemap that it constantly returned an error 500. So I tried a few other scrips and they all produced an error. But when I visit the page in a browser it looks fine.

Where do I start?

Thanks! I used the steps in that forum to go through my plug-ins. It seeps FileDownloadPlugin was the cause. Now, how to fix it? Any suggestions?