I guess the place to start would be with Webfusion, then. Server 500 error is a scripting error...typically...but can also show up with other things when the server is not configured properly. You need to contact your host..no need to try and fix something they should.
I'm not saying we won't help..we just need more info!
Has it always been like this? What did you install that started these errors? Linux or Windows vps? The problem is simple..error 500 covers a multitude of server config errors...and there is no way to figure out what is wrong except with more information.