Answer:
Please note the following important points:
- Include the correct latest links/references, for example, Dlubal.RFEM5.dll, under the following path: C:\Windows\Microsoft.NET\assembly\GAC_MIL\Dlubal.RFEM5\or Dlubal.STEEL_EC3.dll under the following path: C:\Windows\assembly\GAC_MSIL\Dlubal.STEEL_EC3\
- In Visual Studio, the properties "Embed Interop Types" and "Copy Local" must be set to "False" (see the image).