>This error appears after compiling an app. The app contains a couple of classes and references to FoxTools.fll and a 3rd-party .FLL
>
>I've tried backtracking, removing references to specific classes and the 3rd Party FLL, but no luck.
>
>Does anyone have any suggestions?

It sounds like something is corrupted. I would start with recreating the project from scratch and see if it helps.