"It's never happened before" suggests that you have made Colloidal Silver successfully on other occasions.
Assuming your approach has stayed the same, your recent situation of 'fermentation and a putrid smell' suggests that either the container still had some residue (not completely sanitized), or there is some contamination within the golden syrup or electrolyte.
If the golden syrup is old, perhaps it is time to make new. (Fortunately this is not terribly expensive!)
From my experience, I've never had a problem with my electrolyte being either contaminated or 'too old.'
Just my two cents!