Almost without a doubt it is an ev100 controller, (unless the truck is so new this could possibly be a warranty repair) and the code 16 is that the controller is set for either dual voltage and your 36volt battery is just off the charger, or too low a setting for the actual battery voltage.
The "-16" SHOULD indicate too high voltage in the battery, and would (correctly) happen if the controller was adjusted for a 36volt battery, and you installed a 48 volt battery, but in your case, my bet is that the battery is "too hot".
The correct fix would be to get your battery checked and fixed by a good battery company (especially since you have observed the "dropping out under load"), and to have someone with an ev100 handset to adjust function setting #15 to the actual 'at rest' voltage of the battery.
As an emergency workaround, turn on the lights and let them sit with the lights on for a couple of hours, then unplug and re-plug in the battery and turn the key on. this should drain some of the "surface charge" of the battery, and allow the controller to see a more accurate battery voltage.
You may be able to just do one or the other of the suggested "correct repairs" and have it work, but it would be far better (for many reasons, we can discuss if you want) to do both repairs; battery and adjust function 15 on the controller.