From my experience (owned a 14c 680wh myself) and especially all i heard from a very big rider group in asia with very much! Ks14c owners:
The 14c is known to be not able to go 30kmh over its complete batterie life! or better say...if you draw to much power in the second half of batterie...a cutout CAN happen...if really all negatives comes together(weight, hill, batterie percentage, wind etc etc)
As you are speaking of a uphill AND accelerating...this seams to be thecase here!
IF you have a newer version of 14C, which is with upgradeable firmware, do yourself a favor and update to latest firmware 1.25!!!
This firmware will prevent you from speeding to high on a certain batterie percentage! If it is not upgradable...dont go higher than. 25-27kmh when50% and below, and dont do hard accelerations!
AND yes...some here have never got problems with it...but believe me...its a known fact.....