Hi guys, i just bought a refurbished rancilio tecna de s20 but once connected to water inlet and turned on, there is a leak (see picture). starting as a drip, it becomes a stream quite fast... is this a leak at the pump gasket?

I would believe that it is leaking where the pump itself attaches to the motor (pump seal). It would be wise to have it taken care of before the motor gets damaged from water exposure. You state it is a refurbished unit acquired recently ? Most decent companies will provide some sort of short term warranty. Have you contacted them with your issue ? If you have to take it apart- then a pump rebuild would be your best bet. Send it out and get some sort of warranty on the rebuild if you have to.

Is that not the pump mounted to the motor? If steam comes out there there is certainly a problem.
I checked some other images of that machine (see THIS thread here on HB as an example). If that is like yours, it appears that there is a solenoid valve mounted directly to the pump shown HERE. This should keep steam and water from heading back through the pump (a guess, but seems to make sense). IF that is the case, then the valve needs to be replaced or maybe just cleaned. The heat may have damaged the pump as it shouldn't leak there under pressure the boiler creates. It should hold well over 10 bar.