500 errors commonly happen when you have permissions problems or other security problems. ie: is your IIS set up to reject certain permissions levels? are certain folders too low permission, or too high permission for how the server is configured? Your webmaster should know what the restrictions imposed by the server are set to.

Also, some versions of IIS don't like .htaccess files, and in such case you'll have to build the security protections into IIS after removing the .htaccess files that are confusing it. Work with your hosting company or your own security/IIS professional to figure out how to do that on their servers.