It appears to me that IC1C does nothing. R7 and R5 inputs are tied together so there will never be a potential difference to give IC1C and output. Or am I missing something?
A pull-down resistor did the trick. Please feel free to use this circuit. Just give credit where it's due. If you have any modifications to it, please post here so everyone can benefit. This is the first complete RTD pt100 input circuit I've seen in the public domain which includes a current source and output-scaling. There are others out there which do different parts better, but not all together like this.