Do you have Autodesk.AutoCAD.Interop/Autodesk.AutoCAD.Interop.Common.dll referenced in the project? If not, you should have, because all the Civil3D COM interop DLLs are dependent to the vanilla AutoCAD COM interop Dlls.
Or, maybe this:
Because VisualStudio (since 2012?) automatically embeds COM interop types in the project, so, it is possible when you upgraded your project from Civil2016 to Civil2018, VS did not create a new project file (and its resources), thus the project somehow inherited the embedded Autodesk.AutoCAD.Interop/Common dlls? If it is this case, the later version of Civil COM interop DLLs may not be compatible to early version of AutoCAD interops.
So, you may want to try to remove ALL COM interop DLLs (Civil3D's and AutoCAD's) and re-add them back with sure that the COM interops all being from Civil3D2018.