I have a BlueSolar MPPT 150/70 with 2.7kw of panels, this means now thats its sunny in the morning it will run at full capacity, 70A until it fills the batteries, this is like 3h, reaching 40c with external fan or up to 60c without, and no derating.
Is this normal? Can bad terminal connections cause this? Is there a way to measure if something is wrong?
They run quite hot, this is normal. Even with 99% efficiency that would mean 27 watts being lost as heat which is why they need to be mounted vertically so the fins can dissipate the heat by convection.
IIRC they have internal overtemperature protection so will shut down if they get too hot.
It is always a good idea to check the tightness of the connections once or twice a year, as the thermal cycling means they can work loose and there are a few examples on here of failure due to overheating of the terminals.
