500 response code error in my wordpress MU

I am running a WordPress Multisite and the main domain shows no error.

Actually the main domain displays some teasers from the sub domain and click the link will take to the main content in the subdomain.

I am getting a lot of 500 errors in my google webmaster tool.

Something like this

http://tinyurl.com/l9cqzkm

http://tinyurl.com/mfc657t

And when i go to the link i get the 500 error. If i remove the index.php in all the error links the page loads.

Any help will be appreciated.

Thanks