My versions are:
I am unaware of any software issue, but the origin most likely is in Mono (what the message says) - note that @Billy had not upgraded Linux at that time - or in Visual Studio C# which of course cooperates with the runtime. CumulusUtils seems to be running fine and until now the modules and/or website are generated correctly as far as I can see.Raspbian Buster / 5.4.51-v7+ armv7l (before: 4.19.118)
Mono JIT compiler version 6.10.0.104 (before: 6.8.0.123)
I assume there is no issue on Windows (native .NET).
I will not generate new releases until I know more about this stackdump of Mono and its effects.
I have no idea of its effects on CMX itself.