On the refill 100 and 125 machine, this is the drain plug to allow draining of the boiler. Although this is generally not the issue if a slight amount of steam or water is leaking from this plug, periodically, it may be the plug if it’s tightened too much after draining and flushing. Usually, its the o ring that leaks, but this is the drain plug

It comes with a new oring. Replacing should be done ONLY with a cold and nonpressurized machine. DO NOT overtighten.