MarkovicD on "Polylang working except for Serbian language (500 error)"

My blog is in English, Greek and Serbian thanks to Polylang. Lately I encounter a problem only with the Serbian pages though, which show a 500 internal server error "The server encountered an internal error or misconfiguration and was unable to complete your request."

What could be the cause of this error?

http://www.thesouffleur.com/

http://wordpress.org/plugins/polylang/