6411
I am getting it very often on one of my Websites. It is a pretty heavy Website with a lot of modules. But I didn't have the problems, till the vendor (IPowerWeb) switched to new servers. Since then I am getting the same errors as you do - I believe that they just overloaded the servers and shorten the response time, and after 10-20 seconds of waiting for finish queries on the server, the server just gives up and displays 500 Error.
Therefore I am testing now the same website with another vendor, as I am thinking that I will need to switch, as the support on IPowerWeb is not able to resolve it.