The manual only says something about not using Lithium batteries.
Do not use lithium cells! Their higher voltage would damage the flash
unit’s electronic system. Only use the permitted power sources (see
above). Warranty claims for faults and damage to the flash unit are not
accepted if they were caused by the use of accessories from other manufacturers.
It says that in every language included in the manual.
NiCads are 1.2 V, opposed to regular alkaline battery's 1.5 V.
So no problem.