Hi WebmaisterPro friends,
I run on this error several months ago, but I see that today many people still run on it after Kloxo update done in Kloxo panel.
The main problem is that most low end VPS companies are keeping old templates of "host in box" and Kloxo will crush around v6.0.7 as all updates are applied one by one, before you go to latest version.
So, if Kloxo have already crash, you are unable to login and you see 500 error which still remains even after restarting the VPS, then you need to do this to fix it.
SSH the following:
service xinetd restart
It should take less than a minute and everything will be back, don't worry you will not loose any data.
Remember, it is always better to update through SSH command than with control panel, it is easier: