I gave the smaller crucible a try though (empty), and it presents a very light load. At 12V / 50% duty cycle it takes ages to heat up and only gets to the point where it is glowing a bit.

Pardon my butting in on this thread since I have such limited experience.
But---   
With my 2500 watt Chinese induction heater running on 48 volts (at, I'd say, 100% duty cycle)  I'd draw about 35 power supply amps with a crucible of about that size in the work coil..  Subtract about 5 amps for what I call "idle current",   and I have about 30 X 48 = 1440 watts going into the crucible.  Within a couple of minutes, it starts to glow and within 10 minutes it is glowing a very bright orange, even with a couple of hundred grams of copper scrap in it.
So----  you are at about 1/4 of that input voltage and only half the "on time", for about 180 net watts into the work,  so I am not surprised that you don't see much heating going on.  If I were you, I'd get that supply voltage up as much as you can.

What is your goal for melting metals?
-Ferrous?  Non-Ferrous?
-Weight?   100 grams?  1000 grams? 
-Melt time to pouring temp?   I minute?  1 Hour?

Are you measuring power supply input current?  I'd suggest that you do that so you can get an idea of overall circuit efficiency.

induction is cheaper to run. I think.

No.  I tell you again:  A watt is a watt.  You will gain nothing for all your trouble.  In fact, if you intend to use the device to heat water, you will waste power in your power supply and in circuit losses that wiil NOT heat your water.
 
If your goal is to heat the water in an existing system, you can use inline electric water heaters like this one:
https://www.fleetfarm.com/detail/kat-s-circulating-tank-heater/0000000047511?Ntt=005101472&gad_source=1
I am certain that there are MANY similar heaters like this available all around the world, and they won't require any ancillary equipment or electronics.

Looks like it started leaking there at the end of the video.

I think I'd be worried about safety with that project.

Maybe the easiest answer for Alberto is to NOT modify the original circuit at all.  (No separate gate power supply at all)
It has worked quite well for me for several years. as long as the main DC power supply can  deliver at least 20 amperes or more at his 30 volts.
In my own experience,  the gating problems seem to be related to situations where the DC power supply's voltage drops below 12 upon startup.
I usually switch the DC power on with a DC rated circuit breaker AFTER I am certain that the power supply is up to full output voltage.

Refrigeration and air conditioning companies seem to use these fittings.
I convert my systems to 1/4" OD copper tubing and fittings for that exact reason.  (I am in the USA)

"No matter in any case, no-load is allowed (no-load refers to the output port without any load).----

They mean that you must not apply power to the ZVS circuit if you do not have a proper work coil connected.
As you have already observed, there is NO PROBLEM with having the unit powered up with no work piece in the work coil.

Second question:
For the same reason as first question I would like to use a footpedal to engage heating. Or maybe have I misunderstood it and there is nog engage exept turning on the machine?

The "footpedal" can be used to trigger an appropriately sized mechanical relay or an appropriately sized DC Solid State Relay.
I assume that you will already have your work piece in the work coil. I have tested this case and I see no problem with it.
---As long as you already know that the current that will be drawn by that work piece will not exceed the capabilities of the relay or the ZVS board.
In this case, of course, you will need to have your DC power supply "ON" and only use the relay to send the DC  power to the ZVS board.

If any of this is unclear, please feel free to ask for more information.  !!! Your English is very good!!!

Thank you for asking!!!
I am the original poster for this thread.
   It still gets between 50 and 100 views per day.
I consider it to be a resource to those just getting into induction heating with the 1000 to 2500 watt or so induction heaters sold all over the world.
I know it takes a while to locate specifics by scrolling through all the information here.
So, Here are a couple of links links that take you directly to my many  youtube videos and webpages on the subject;
https://www.youtube.com/playlist?list=PLVxWen9M87dBhvInCQ-3pELWdxCM3XH4I
https://spaco.org/Blacksmithing/ZVSInductionHeater/1000WattZVSInductionHeaterNotes.htm
I am certain that I have posted them previously here, but this is a quicker way to get started.
I don't post testing and communications here where the folks who write to me asking for consultation on projects that have proprietary concerns.
But I do have to say that, over the last few years I have seen these small induction heaters used on some pretty interesting  "out of the box thinking" projects. 

Re:  "who else has stories ---?"
Galloping Power lines:
  We have had a few instances of galloping transmission lines (150Kv and 300Kv lines run along one edge of our property about 1/4 mile away from our home).
See this for an explanation:
https://www.lanescott.coop/when-winter-winds-howl-power-lines-can-gallop
About 10 years ago the transmission line companies began winding some heavy gauge wire around the lines in certain areas where the high winds are prevalent.  They would wrap  10 or 20 foot long sections of the line, skip 20 or 40 feet and then do it again.  The idea is to  destroy the airfoil shape  of the wire to prevent the "galloping". 
    Almost off topic, but of interest to me:
About 5 years ago, one of those transmission line companies totally rebuilt their power line.  In so doing, they cut back the trees in one portion of our woods, all the way to the very edge of their easement. (In the past, they had not gone as far as they could have).   Anyway, 30 or more of the trees they  dropped were in the 30 to 50 or more years age group. All hardwoods like Maple, Oak, Hickory, Ash, etc.. I made about 30 logs from them and had about 1500 board feet of lumber milled from all that.
See:
/>https://spaco.org/Forestry/SawingWood2009.htm

- please see (11).

- thinking i would be able to measure current.

Re: please see 11:  Ahha- so the highest voltage that was ever applied to the system was 32 volts, right?
Well, it MIGHT be that the work coil heated up so much that it tranfered too much heat to the pcb and its components.  Was the work coil discolored when you returned to check the system?
Otherwise, sorry,  but I am stumped. I wonder if you could apply with the seller to return it under warranty.

Re:  thinking---:   Was that loop part of the original circuit or was it something you added for testing?

Re: "book to learn electronics":
I too, learned electronics a long time ago, starting in about 1948.
If I were starting out today, I think I'd get one of those "learn electronics kits"

Here's one example:
https://www.sciplus.com/electronic-playground-48491-p?