@martinsiegrist I think you might be running Rhino 9 WIP in .NET Framework (4.8) mode, and Isopod is only compiled for .NET 7. If you use SetDotNetRuntime, you should be able to switch it back to .NET Core mode which should get this working again. If you’ve modified your shortcut to add /netfx as an argument you’ll have to remove that.
It might be worth it to get Isopod multi-targeted so it can run in either framework.