"Carbon Black Cloud", that's the reason. No matter what compilation options I tried I never could manage to run my little innocent app in the client's server without this shitty "protection" software blocking it.
In the end I had to redo it in C#. Luckily it was mostly a line-by-line translation with the occasional workaround. Let's say I have now a better appreciation of UFCS, templates, strings as value types and terse syntax in general (but I still prefer block end delimiters)... Anyway, is there any way to deal with confused "antimalware" software? Have the Nim team talked with them? Obviously they aren't looking in the right place in the binary to decide whether it is malware or not.