Access to the path "/etc/mono/registry" is denied.

[se5a]

Ncalc calls System.Diagnostics.Trace.TraceInformation(string message) which eventually tries to create a directory in the registry via System.IO.Directory.CreateDirectoriesInternal(string path) somewhere along the way it decides whether to use HKCU - which will work fine as that writes to ~/.mono/registry, however in some cases it'll go for HKLM which is in etc/mono/registry. which the user doesn't typically have access to (without resulting to sudo)

https://github.com/mono/mono/blob/master/mcs/class/referencesource/System.Xml/System/Xml/Core/XmlReaderSettings.cs#L734

you can see the callstack and showing external code to see this when it happens.
I don't know why this happens on some systems and not on others.
One work around (which is the wrong fix) is to create the folder in etc/mono/registry and set permissions so the user can write to it. until I figure out what's really going on with this it's all I've got.