Difference between Proteus results and KCL-deduced formula
I've found a pretty acceptable solution.
The formula for the output voltage of the diagram above is provided in another question, I've recently posted:
Output Voltage Formula verification of Operational Amplifier circuit.
The diagram can easily be simulated in Proteus and provide a temperature range between 0 and 400 degree celcius.

Where Rx=R6=Resistance of the PT100 sensor.

The problem I was having was that I was wrongly connecting my PT100 (3-wires) RTD within the circuit (the above circuit assumes a 2-wire RTD).
The other mistake was to use a 5V supply source for the Op-Amp: no, the voltage source for the op-amp needs to be at least 6V. Preferably, the op-amp should be powered by a 6V and -6V as I still get wrong results with temperatures below 50 degree Celsius.
In my case, I'm mostly concerned with temperatures above 100 degree Celcius. So I'm fine with only a 6V supply for the Op-Amp.