An E-load will not resemble a LED string, much. And a
mess of LEDs, perhaps with added relays to control
load current, ought to be a factor of 100 cheaper than
a good quality E-load.
If this is a closed-loop load that tries to enforce a current
then its frequency response could be fighting with the
loop response of the source.
I have had trouble with E-loads in power supply testing at
low voltages, they require headroom that can't be had at
1V and a few meters of cabling. Some of them have devils
in the spec details.
For simplicity's sake you might try a resistor load of roughly-
right value, to see which end is having the problem.