Internal Server Error After Creating New Site

Last fall I started setting up my new WPMU network on my VPS. After debugging the mapping plug-in and some settings on the server, we got five sites up and working and they continue to do so, although they're still under development. See post 310311.

However, I've acquired some more domain names and went to set up some more sites on my server.

The initial "add new" site via the WPMU network dashboard appeared to work OK. However when I try to access the newly create site's own dashboard, I get an "Internal Server Error" message with a secondary 500 server error.

I also tried to access the full URL and the .com URL for the blog, and for both I get the same "Index of /" displayed with cgi-bin/ on the next line.

My hosting provider's excellent technical support folks are still looking into it from their end, but so far they believe everything is configured properly. We have the working sites to compare the settings with.

The only changes I've made since setting up the first sites is the regular WP and WPMU updates.

Does anyone on this forum have any ideas?

Thanks