Cannot submit some forms

After submitting i see a blank page with a small window, containing this text: { "status": true, "updaters": [ ], "debug": [ ], "messages_error": [ ], "messages_status": [ ], "messages_warning": [ ], "redirect": "http://fffoto.ru/node/29?114383958=1", "preview": null, "form_id": "comment_form", "options": { "enabled": true } }

Some forms work correct, but other (adding comment, for example) don't.

See attach for details.