Dear Charl,
Thank you for the reply.
I finally managed to figure it out a couple of days ago!
It was actually a negative synaptic weight causing the problem (as the message in your response implies).
Sorry for providing such long codefiles. The code was originally written for Nest 2.2 and I did several modifications to update it to 2.18. Apparently, negative weights were allowed back then. In my case, I missed this minus sign and for some reason it was very hard to pinpoint where the problem originated without looking under the C++ hood.
Best
Angeliki