It turns out there was a conflict with an old version of SpeckleCore.dll (and other dlls too) loaded by an internal plugin!
Removing it resolved the issue. For others having similar problems you can try the following:
- create a c# node
- add this at the top
using System.Linq; - add this to the function body
A = AppDomain.CurrentDomain.GetAssemblies().Select(o => o.FullName).Where(o => o.Contains("Speckle")).ToList();
And then show us what the output is! 