Unable to load Project_DEBUG.dll

Recently the following error has been popping up when trying to build a project I'm working on:

"Unable to load game library: Path\Project_DEBUG.dll Please make sure it exists and the latest DirectX is installed."

We can't track down whether it is something in the project settings or whether it is something in the code that is causing the error, and it seems that sometimes a clean and rebuild will fix it.

Any insight into what may be the cause of this would be appreciated.

Thanks.