Solution
There are 2 possible reasons you might be having this problem:
1. Your assembly can't be loaded as it is missing dependencies. To fix this ensure all
dependencies are referenced correctly.
1.1 If the issue happens trying to open some third-party .NET control, some of these controls will require the .NET driver code to be in the same folder as the LabVIEW project code. Some .NET controls will allow the .NET driver code to be in a LabVIEW Project subfolder
1.2 If the issue happens trying to open some third-party .NET control in an executable, some of these controls will require the .NET driver code to be in the same folder as the executable.
2. You are trying to load a .NET file of a different bitness to the version of LabVIEW you are running. To fix this ensure you are trying to load the file using a LabVIEW version of the corresponding bitness.