This error is a little tricky. I think a big part to solving this would be in checking when you get the error. If you get the error while building the project, it is more than likely a linker error. If you get the error during runtime, I would guess it to be an execution error in your DLL.
If it happened at build time, I think you must be linking to the library or whatever from your dll that isn't correct, and so the linker errors out. If the error is during runtime, I think that something has happened in your DLL or your definition of the function that has caused the engine to error because it doesn't find anything. Maybe you renamed it, or typed it wrong or something along those lines.
