Finally I managed to solve this.
The problem that the cron event “nxs_querypost_event” (which is the one that activates de autoreposter) didn’t exist, so I had to create it. It’s easy to create cron events with the plugi Wp-Crontrol.
Also, my theme (ionMag) was blocking the non-default cron-scheuldes (I could only select every hour, day, week or month), so I has to change to another theme so I could scheulde the event every 15 minutes.
SOLVED.
