When setting a non-existent property on a large number of synapses, the memory consumption of NEST explodes and the process soon crashes. Minimal reproducer:

import numpy as np
import nest

pop = nest.Create("iaf_psc_exp", 160000)
nest.Connect(pop, pop, "one_to_one")
connections = nest.GetConnections()
connections.set({"awefwef": np.linspace(0,1,len(connections))})  # crash here

This is working totally fine for 16000 nodes on my system and gives the following (expected) error message without delay:

NESTErrors.DictError: DictError in SLI function SetStatus_CD: Unused dictionary items:  awefwef
Maybe you tried to set common synapse properties through an individual synapse?

If I change the line to

connections.set({"awefwef": 42})

the issue also goes away, so having a large data payload seems to be part of the problem.