Hot to remove the /blog from URL in a multisite install

My install gets a /blog in the URL and i cannot remove that in the permalinks structure.

It's only in the main blog, and I removed it manually, editing the configuration in NETWORK ADMIN>>SITES>>MAIN SITE>>SETTINGS, but tht's causing a few problems with some plugins, and i get a 404 error.

Does anyone know how to solve that?

I'm using NginX as websever and I don't know if that's causing it.