Hello and thanks for the replies. The problem is now solved. I changed the theme (Twenty Fourteen) and deactivated all plugins. the feed is ok. I reactivated the plugins and had the same error. Are returned to the previous theme with all plugins disabled and the feed works.
By enabling one at a time I realized the plugin which gave problems (yakadanda instagram)
I deactivated the plugin and now everything is ok.
Thank you very much.
