I cannnot install themes nor plugins

http://lightcanvas.net

When I try to install a theme, it gives an error and I don’t know what is causing this problem. I have contacted you regarding this issue before. And I found “installer” plug in was causing a problem. But even I de-activate it the site still doesn’t let me install or update.

I de-activated most of the plugins except the plugins from WPMU dev. I changed my theme folder to lc-website from wp-content. Is it causing this issue?

I granted support access already, so please take a look.