Water is leaking from the side of the machine, at the point where the boiler and grouphead join, as shown by the arrow in this picture:
If water is leaking from this point, it’s coming past the upper chamber o-ring. If this is the case, it’s most likely that the boiler simply hasn’t been screwed on tight enough. Simply tighten the boiler more next time you use the machine. It’s also possible that the boiler o-ring has popped out of its groove, so it’s a good idea to check that too.