The obvious answer is that the overheat thermostat or the thermostatic mixing valve (TMV) has failed. The overheat stat operates to prevent water of more than 55 degrees C entering the UFH system - but the flow temperature limit setting is adjustable at the controller so could be significantly lower.
The TMV limits flow temperature by blending the incoming flow temperature.
However, from the description of your first post you seem to be getting flow across the manifold when the UFH pump is off - you said that the top branch of the manifold is hot but the lower branch is cold. That seems to indicate that the control valve is passing and that you are getting a secondary reverse flow of unblended boiler flow water. If that is the case you could get water at a higher temperature than 55 degrees (or whatever the overheat stat is set at) reaching the overheat thermostat, which could / would trigger it to operate.
The control logic for your system to operate correctly is that the control valve on the lower right of the manifold opens to allow the boiler flow to enter, it passes through the thermostatic mixing valve with the external temperature sensor and blends with the return water from the UFH, it reaches the pump, is pumped and passes though the overheat thermostat (limited by its setting - see above) then enters the UFH system - which can be seen by the balanced flow valves, after the last balanced flow valve has been satisfied the flow becomes the return, which should be direct, back to the boiler.
In essence if you are getting seepage from the control valve, with the Wilo pump not running, the lower part of the manifold should be hotter than the upper part.
In your case there appears to be an installation configuration issue that may be allowing a secondary reverse flow when the UFH pump is not operating
If everything is operating correctly, when the Wilo UFH pump is off ( after a reasonable period of time) the top and bottom of the manifold should be cool.
You have probably not got complete hydraulic separation between the two pumps - replacing the thermostatic mixing valve and reconfiguring using an injector valve would resolve that.
Hope this all makes sense