Session state can only be used when enablesessionstate is set to true

I had a JavaScript function, that called a WebService, and I had this "Session state can only be used when enablesessionstate is set to true ..." error over and over again.

Everything was all right, in the web.config and in the "< % @ Page " too.

It was still giving me the error.

I made it work doing this:
start -- control panel -- administrative tools -- services -- asp.net state server

by default it is stopped. i started it.
that's it.