Ok for anyone having the same issue, the problem is that the heating barrel at its back end is thin and does not have enough mass to melt any solder stuck there.
I removed the waste barrel and saw a blob of solder sticking out from the back end.
I cut it with my cutters as close as I could without damaging the heating barrel.
I also removed the black seal and I was thinking to heat the back end with my soldering iron to let the solder run towards the front releasing the hole.
But as it seems, even leaving it like that for a minute or so is enough for the solder to melt just with the heat from the gun at 350C. It was probably the blob that was causing the problem.
Just be careful not to burn your hands handling the barrels
The gun is running perfectly once again.