I use a voltage divider (110K - 10K Pot) to measure 0-60V.
At low voltages it works very good but as I increase the voltage the error increases. At >40V the error starts to get big (several volts).
I would not discard heating: although the power you generate is less than a mW, common resistors and pots are very sensitive to temperature.
Does the measured voltage vary as time passes?. Does it stabilize finally at a -wrong or not- value?
Regards