If you click the link at the top of a blog post that says from So and So's blog to see all that persons blog post it gives a 500 error when your not logged in. If your logged in it will take you there with out any problems. I don't know how long this has been going on because I'm usually logged in. I'm using version 1.5.3 (build 6341) and I'm not sure if this just started after I updated to this version or not.
Thanks Abbey, I tried that, I disabled plugins all the way down to where I only had the blog plugin and it still did the same thing. I'm thinking it has something to do with permissions because it works if your logged in.
I'm saying its a 500 error Internet Explorer says "The website cannot display the page HTTP 500", Firefox and Chrome display a blank white page which is usually a php error.