using an "isolated DC/DC converter", what is the schematic?
Assuming you've actually connected pins 1 &amp; 2 the correct way around, then have a look at the 'Output Voltage Tolerance Envelope' graph at the top of page 2 of the datasheet.
You'll notice that it doesn't show anything for an output load current of less than 10% of its max rated output.  

I take this to mean that this DC-DC converter needs to have at least some load connected before it will correctly regulate its output voltage.  

In this case a 470 ohm resistor should just suffice.