Isolated DC-DC Converter screws up ADC

I am developing the automated test system depicted in the following picture.

1. The system receive commands and returns result data via Bluetooth to a remote computer
2. DK52 is used to fine tune the control software and will be replaced by a custom board based on nRF52832
3. Control Circuit generates testing signals according to the received commands
4. ADC measures voltage applied to the DUT and drained current
5. To respect the ADC specs the two circuits are isolated by ISO7741

Everything works fine.

To simplify the system I replaced the power supply 2 with a isolated DC-DC converter (DPAN02A-05) connected to power supply 1.

After that the current measurements (differential voltage across R1) are completely screwed up. Measured current is definitely lower than the correct value and it is negative instead of positive.

Conversely, voltage measured across DUT is correct (even if a little bit lower than the expected value. Maybe because the supply voltage is noisy since the DC-DC converter).

I measured +5V and +3.3V and they are correct.

I have been bumping my head on the wall for days trying to figure out what’s wrong.

Please, give me some help!