The thing with a fishless cycle is that you are supposed to be able to fully stock in one hit, but I'm not 100% certain that it was done completely correctly - once you had removed the ammo-lock, I think it should have taken another 3-4 weeks to fully cycle. The Nitrite munchers in particular take a while to establish. So I think TnT is kinda right, without realising it.
But I do think you are rather overstocked for a 60l tank, yes, and that may have overloaded whatever bacteria you had there.
Keep testing daily, and changing water if you see +ve results for ammonia or nitrite.