i tried this solution based on the article rmartine gave.
1. i removed the end slash at the
Alias /doc/ /usr/share/doc
2. But because my server is a local development server, thus the servername is not FQDN, i need to remain using IP addressing. (http://161.x.x.x.x/site1). So i turn off here
and it works. I dun know if this is a valid solution, but it works for me.
Btw, my DocumentRoot is already set without any ending slash.
thanks guys for ur help.