I did a lot of teeth gnashing on that one. The problem that I found was the Internal Error 500 is being generated by Apache, and never makes it to a log file. What I found was if I couldn't get to a directory correctly because of the Apache security (shared host) it was bombing. I eventually called my hosting company to figure out how to handle a subdomain to subdirectory properly and that allowed Silverstripe to run properly. I have seen some issues with the Subsite module, but I need to update my SS and subsite to the latest and greatest before I complain too much.