For those with C++ understanding, running the project through Visual Studio will trigger the ensure() function and cause a breakpoint. By going back through the call stack and viewing the input variables of the blueprint classes, you can read the name of both the calling class as well as the Blueprint Function Library containing the __WorldContext parameter that shouldn’t be there (there’s also a “then” parameter which you can ignore).
(@Willard720)
Simply launch the engine, go to the found Blueprint Function Library, and remove the __WorldContext parameter from the function. Nofear001 was correct in saying that this is caused by copying an existing function inside a function library. Removing the __WorldContext input pin from the function will remove the error and cause the succesful opening of the project, without the ensure() function notifying Visual Studio, as well as a succesful compile.
Tl;dr: __WorldContext pin needs REMOVED, not filled. Calling the function has 0 WorldContext pins, Unreal does this for you under the hood.