I was thinking of trying new Thermal paste.
I've searched around to figure out how the liquid cooling works and what is the actual "liquid" used. So far I haven't found info on it.
The hypothesis is that maybe it is leaking or evaporating because at least in my case the problem has been gradually increasing. For a while putting a fan on the back fixed the problem, but this fix is no longer effective for me most of the time.
It wouldn't be hard to make a small hole in the copper pipe at the right place, add some liquid and solder the thing shut again but which liquid and how much?