Hi,

false alarm. Shame on me!

The problem was that after removing the radiator to do the valve clearance, I fit two hoses in the wrong place. Basically, the hose that communicates the expansion tank and the rad was in the wrong place. And so was the one that return the coolant into the engine (is that the purpose of the hose just under the neck of the rad?).
Since I only run the engine for a couple of minutes with the hoses on the wrong place I think the mistake did not have further consequences. I run the bike yesterday after fitting the hoses correctly and it all looks good.

Thanks for the suggestions Pleiades.